A standard Ashkenazi kesuba is a document no different to a post dated cheque; there is nothing religious or holy about it. It says that the man will pay his wife upon divorce or death the sum of 200 silver German marks (or zekukin). This amounts to 100 lbs in total or 230 grams per mark. This was instituted when the Jews lived in Germany most likely in the times of Tosfos, nearly one thousand years ago. See my comment that unlike RMF it is more likely the French mark which was 244 grams and more than the 100 lbs quoted by RMF.
At the time, everyone who got married in Germany would know exactly what was written there, like today everyone knows what dollars are. Later as the Jews moved eastwards it was considered too much and the women had to agree (reluctantly!) to forgo quite a large part of it, like in Russia where she only ended up with roubles to the value of forty dollars. This in no way means that the amount the man offered changed at all but only that she couldn't collect.
The Shulchan Oruch in CM writes regarding a shtar (document) that if the man does not know what is written there (and you believe him) he does not have to pay it. It gives the example of a Sefardi Kesuba which does not have a standard amount. The Chochmas Shlomo at the side says it depends if he himself signed it or not. A man can sign a paper and let someone else put any limitless amount he fancies, but not witnesses.
Today when not everyone reads this blog and very few people know what the 200 zekukim are, even rabbonim, much less the value of them being 100 lbs of silver, how can a kesuba be kosher? If it was a shtar it would not be unless it was signed, which not all kesubas are. Just because it is in a standard form should that make a difference? As I have proven, it was not always like this, since when it was instituted in Germany everyone knew what a German mark was.
My second question is: Rabbi Moshe Feinstein says the custom in Russia was to give the value of forty dollars. Since there doesn't seem to be any custom today, what right does an individual bais din have to decide for themselves, often based on the couple's worth, of how much the woman has to forgo?