Kevin Costner's divorce sheds light on the complexity of a celebrity split. A matrimonial lawyer shares what goes into prenups, postnups of the rich and famous.

Kevin Costner settled his divorce battle with Christine — and while there are no specifics, beyond it being called "an amicable and mutually agreed upon resolution" — she reportedly received a payday she's happy with.

When the Yellowstone actor said "I do" to his second wife in 2004 (after his first divorce socked him in the wallet), they had a prenuptial agreement that stipulated she would receive $1.4 million if they split. His homes purchased before their marriage would remain his and their earnings would stay separate. He'd kick in on the first year of her mortgage payments and property taxes when she moved out.

Only, his wealth ballooned to an estimated $400 million during the time they were married, thanks in large part to Yellowstone, and they lived a lavish lifestyle together. Plus, the couple had three children and stayed together for nearly two decades. So she seemed to be making a play for more, in addition to child support, before they came to the settlement. However, Christine risked losing the $1.4 million Kevin already gave her by challenging the prenup in the first place — and potentially had to pay his legal fees (estimated at $850,000) in addition to her own (she owed $300,000), which would have been all the money.

What's a prenup — and who gets one?

It's a legal contract made between two people — one of whom likely has more assets or debts than the other — before marriage that establishes rights to property and support in the event of divorce or death. It's a roadmap for what happens to the money/property should they divorce, but also a plan for assets acquired during the marriage (whether they should be considered separate or joint). A prenup must include full disclosure of all finances. The parties need separate attorneys and it shouldn't be signed at the eleventh hour (because a judge could feel someone was pressured). It shouldn't include any child custody or support details. Some prenups have termination dates (e.g., the 10th anniversary).

"Everything has to be dotted and crossed," matrimonial and family law attorney Nancy Chemtob, founding and managing partner at NYC law firm Chemtob Moss Forman & Beyda, tells Yahoo Entertainment. "You can't just do it on a piece of paper. It has to be notarized."

In exchange for signing a prenup, there's consideration.

"I'm gonna give you something for signing this — because you're taking away someone's rights in exchange," explains Chemtob, who's helped Mary-Kate Olsen, chef Bobby Flay, designer Tory Burch and other celebrities get divorced. "For instance: 'You're not going to get anything from me, but if we get divorced, I'm going to give you $100,000 for every year we're married. I'm going to let you stay in the house until the kids are 22.' If the spouse says, 'I want you to sign a prenuptial agreement and you get no benefit,' that's unconscionable."

Chemtob — who has created prenups for people trying to protect assets that range from $500,000 to $2 billion — says even the "horribly crappy" prenups she's seen give some consideration to the party coming into the marriage with less. It could be an amount of money, jewelry obtained during the marriage or cars.

Cheating and other clauses in prenups and postnups

There's the prenuptial, which comes before an "I do," but its counterpart is a postnuptial agreement, which would be signed after marriage when some marital assets already exist. They're similar agreements, but, from Chemtob's experience, postnups come into play after "something crazy happens" within a relationship, whether it's "somebody cheating" or another reason why the marriage is "on the rocks."

"I have a client — I have many clients like this —where we're on like postnup number five because every time the spouse cheats, my client gets a house, money, jewelry," she says. It's a "way to level the playing fields of bad behavior."

With both agreements, there's maneuvering on both sides.

"Let's say a husband goes on a boys' trip, he's with a woman and she keeps calling, but he promises nothing's happening," she says. "I'll get [from the spouse, her client], 'I really want to get divorced, but in my prenup, it says if I make it to year five, I get $10 million. I want to try to make it to year five.' Or they'll say: 'I want to do a postnup'" to get a new asset earmarked for them in case of divorce. "Sometimes I'll tell my clients: Just pretend everything's great. You forgive him, her. After that, we'll do a postnup and once it's signed, sealed, delivered, wait a few months, then get divorced. You'll get more money."

There can also be angling when it comes to signing prenups. Chemtob says clients have had a partner spring one on them just days before the wedding.

It can get "very tense," she admits. "I'll ask: 'Is he really going to call off the wedding? You have 200 people coming to a destination wedding.'" Sometimes her advice is, "'Just kick the can,'" meaning to put off signing. Sometimes she'll suggest a postnup instead.

Chemtob has been working on one of the "craziest cases I've seen in a while" — a postnup in which her client's spouse "put in [clauses] that are unenforceable. Things that need to do with the child," which can't be included, but also: "That [the wife] has to do certain things. The way she looks, what she wears, things like that. That's unconscionable, so I will make an application to set that aside."

That's the thing about a so-called "ironclad prenup," a term we often hear with celebrity divorces. The ones that actually are ironclad don't include "nonsense" clauses like: "We have to have sex three times a week. You have to wear a red dress every Friday. You're gonna let me go on boys trips,'" Chemtob says, noting, "People put that stuff in there." Those types of lifestyle clauses, including cheating ones, are generally unenforceable.

It's truly ironclad when the agreement is "straightforward" in listing the specific monetary terms: "You get X amount of dollars consideration if we get divorced. I will buy you a home up to $4 million." And when it allows for a cost of living adjustment for each year of marriage. "No monkey business about red dresses."

As for when she says a prenup is absolutely necessary? When someone's going to have a second family. Older people entering a second marriage — who already have money, assets, kids — need a plan for that.

Celebrity prenup trends: Joe Jonas and Sophie Tuner, Sofia Vergara and Joe Manganiello and more

Through the years, it's been reported that different celebrities have various wild prenups clauses entitling them to certain things.

"It's very standard to write: 'If you get married and there are no kids, you get X amount of dollars in a split. If you're married and we have kids, then you get X amount of dollars," Chemtob says. "You just have to stay away from any custody issues, including radius clauses." Agreements can include an "intention" for instance, "'We intend to raise our kids Jewish and intend to send them to a Jewish day school.'" You can't say, "They're 'going to do' that. That's when it becomes messy."

When Joe Jonas and Sophie Turner recently announced their split, a report stated that their prenup allows them to keep the rights to their image, likeness and businesses associated with their celebrity status.

"That's a very, very important clause in a prenuptial agreement of celebrities," Chemtob says. "'Celebrity status' is a legal term in our world," essentially it's anything related to their individual acting, music or celebrity status. "'All of that is mine. You can never get a portion." It's tricky because it's something you can't touch or feel.

"I have this with a lot of tech people," she adds. "It's like a patent. I wrote a patent. It hasn't gone through yet. It's worth nothing. But if my patent comes to fruition, and I [create] Ozempic," for example, "then I don't want you to get any of that money from my patent. It's the depreciation of an asset. I include it with authors. I do it with actors. If somebody came in and said, 'I'm going to be the next Bobby Flay,'" she'd add a clause that the spouse isn't entitled to anything related to future earnings in the cooking industry.

In Sofia Vergara's prenup with Joe Manganiello, the Modern Family actress had a clause about keeping her own art and jewelry. Chemtob includes both jewelry and art clauses in "every single one" of her prenups. She adds, "The one thing that you get no matter what is your engagement ring. That's a premarital asset. The wedding ring, however, is an asset of the marriage."

As for rumors that Britney Spears's soon-to-be ex Sam Asghari may contest their prenup, "All of those one-year, short-term marriages with no kids, I just look at as a long date... [He's] not gonna get anything" more, she predicts.

Costner's "stingy" prenup

When it came to the Costner's prenup — which had a clause stating Christine had to move out of their $100 million-plus family compound within 30 days — Chemtob says in general it seemed "really stingy" considering he's worth $400 million and Christine's payout would have been $1.4 million.

"$1.4 million is a low number," she says. "But when you look at: She had a lawyer, he had a lawyer. For whatever reason, he was like, 'I'm only giving you $1.4' and she agreed to it."

That said, "They've been married a long time. If I were his lawyer, I would [have said] 'Listen, just make a deal.' If I were her lawyer, I would [have said], 'You know what? Right now it seems like you're pissing him off and the nicer you are, the more you'll get. Just make a deal.'" Christine is also getting $63,000 a month in child support (though her rent alone is $40,000 a month).

Chemtob says divorce battles drag out due to bitterness between exes. "When people go to trial," which the Costners nearly did, "they're just really angry."

Having an inside look at these things — "What I see at work is unbelievable, because clients have to tell me the truth," she says — we asked what a reasonable payout to Christine would have been. "I'd say $50 million," she says, noting that it wouldn't really change the actor's lifestyle. He owns two oceanfront properties in Santa Barbara, with multiple homes on them, as well as an Aspen estate.

When we suggest it seems doubtful Kevin, a father of seven through three relationships, remarries after his second terrible divorce, Chemtob says, "You never know," noting that one of her clients once divorced his fourth wife to remarry his third. But celebrity marriages, especially when one person isn't famous, are tough in general, she explains.

"This is what I've realized: The person who's the celebrity gets so much amazing attention. Everybody loves you. They know your name. They want to take your picture," she says. "Then when the go home, the spouse, who is not famous, says to the celebrity: 'Can you turn down air conditioning? Can you get me water?' All of a sudden, they become real people at home. I think that's one of the things that just loses a lot of the luster for these celebrity marriages."