Most entry fees barely cover basic regatta expenses.
It's not easy to find sponsorship for any small cat racing so the money would probably have to come from registrations.
I would not want to pay more entry fee so that the top 5 will get prize money knowing that I have almost no chance of ever making it into that top 5 in any large event.
When you look at a Nationals or Worlds there are only a few people that have a chance to win.

The Sharks in Canandaigua have had a board at their event where you can pay to enter and pick what you think your finish position will be.
If you're right you win. You can finish 52 and be the big winner.

You could just draw 5 random finish positions at the end and award those people equal prize money. This might keep everyone interested and willing to pay a little more.

I would prefer to leave it alone and get my picture taken with the $7 trophey that cost me $1000 to get and be able to look back at it later and remember some good times.