I won a good deal of money racing with the NMRA. Purse was pathetic ($350) , but back in the day contingency was pretty darn good at $150 per, and you could usually have 20 - 30 or so contingency items.
It never covered expenses, but nothing spends like prize money!
Save the receipts for the parts and such you bought for the car. If you want to get really anal,you can keep track of fuel and other consumables too. Usually the receipts outweigh the winnings, and you can declare the prize money to be a "hobby loss". If its not a lot of money, people just keep track of the checks and the parts reciepts if case of an audit and don't file it on their taxes. Apparently with an overwhelming difference between the racing "income" and the expenses, the IRS just waves it by.