This must be the event where I asked what the payout was last year & didn't get an answer. I sorta remember now. In my opinion you have to set a payout before the event if you expect the payout to help bring racers. You can advertise that & such, exactly how much you will pay for each class. This would show much more integrity on your part, rather than deciding how much to pay after you see who shows up. If I take off work & travel 800 miles one way to try to win that $1000, thats what it needs to be whether I'm the only car there or there are 20.
The way you do it now is like telling the neighbor kid to come on over to mow your lawn, which is like 5 acres, so he can earn a bunch of extra money, mentioning that your budget is to spend $12,000 on house chores this weekend. Then after the kid gets done working for hours you say well, its not done exactly how I like it done, but Ill still pay you $5. Then you say I will pay alot more next week if you bring some friends to help.