There are actually a lot of divorces out there that are friendly or at least done on good terms, with little to no problems. Any way it goes, if people end their relationship, it is going to get ugly, whether you have multiple contracts or just a single marriage license. This idea that prenups aren't challenged or iron out all issues is false. Heck, that was one of the major cases in Liar, Liar, the woman challenging the prenup she signed. There is no way to determine what would make a divorce cost any more, but for most people, the highest costs would come from the lawyers, even if they have a prenup, especially if they have all that paperwork done, since no one would be able to really predict their marriage and what direction it could take from the beginning.
Marriage is a legitimate contract right now, one that is very similar to the birth certificate (which is just as much a contract, at least in the agreement that you are claiming that child as yours) or adoption record (definitely a contract saying you agree to care for a child), one that creates a legal kinship first and foremost. Everything else is going to be based on how a judge decides, whether there are additional contracts attached to specific marriages or all marriages, because nothing is set absolutely in stone, no matter how much people may wish to believe that.