Given it was not specified how many entrants the prizes was based on, it shouldn't be changed. People can suggest it's insinuated that less numbers = less money, but if the number of entrants the prize fund is based on isn't given, and changes can be made to the money awarded, it opens a tournament up to give out whatever they like and say they didn't get enough numbers. "We only got 70 entries, prize fund was based on 100".
Getting shortchanged to the tune of $200 for 20th is one thing but shortchanged $330 for 7th won't encourage numbers next year.