Sam: The prenup sets the amount to $55K + inflation (plus her earnings) even if she came from a poor family living in Wisconsin and married a poor husband who earned his living as a cashier in the kosher grocery near a far flung Chabad house.
If they would fight over what the Shulchan Aruch mandated mezonos amount should be, that is what beis din is for – to settle that question. (There is no problem if they agreed to a specific beis din before marriage.) Just because they might fight over the appropriate mezonos amount does not give the right to preset an amount that effectively is a kenas.
(Additionally to the above, even if she lived during her marriage in a fancy Manhattan neighborhood [with an average income that the vast majority of frum men don’t come close to earning], $55K [plus inflation plus her earnings] is well above the amount of mezonos S”A would make him mechuyiv in. S”A doesn’t obligate a husband to provide his wife mezonos to reach “her living expectations”, especially when he cannot afford it. And especially when she left the marital home and he’s, as is his right, offering to cover her mezonos in the home but not out of the home.)