As discussed in the article by Rabbis Reiss and Broyde: worst-case scenario, each party finds some rabbi with some amount, claims that's their rabbi, and we extract the smaller of the claims. (The burden of proof here would be on the wife as she's demanding the money, so if the husband has a plausible source that it's a smaller sum, we'd go with that.) Same as if they signed an agreement to buy/sell a "keg" of beer, then afterwards they each had a plausible source for a different size of "keg." That's still enforceable; this happens all the time in monetary cases where one side claims kim li -- I follow Rabbi so-and-so. (It came up with the different opinions on cancellations due to a pandemic...)
Practically, there is a very strong argument (as found in the article) that if they are two "average OU" American Ashkenazim without a very strong connection to the rabbis with other opinions, the unspoken agreement would be that to default to Rav Moshe Feinstein's view.