First answer: exactly zero! There is no prize money, no guaranteed contract that gives you a car and gas and tires for free. Just maybe if you're really good and win at the SCCA Nationals you might earn a free set of tires and some discounts on brakes if you win on a brand of tire or brakes that sponsors the event. Even if you are the national champion and win a set of tires, I know that you've spent way more than the value of a set of tires on autocross events.
Second answer: Priceless! You didn't get any money for driving, but you got "paid" a fun hobby, lots of hours of meeting other drivers and talking cars and driving, made some new friends, and probably been helped by others who gave you free advice or even helped you work on your car for free. You probably work in the car business or through autocross met a bunch of other people that do, and learned a lot of valuable information on driving, setting up your car, or the name of the best mechanic or car salesman in town.
Third answer: (I'm not good in math, so I lied about there only being two answers) Maybe you won a local event and won a free cup, a t-shirt, or maybe a little trophy or something you can stick in the garage. Even better, maybe you got another sticker you can put on your car, and remember, that every sticker adds 5-10 horsepower! You also probably attended some other car event or car show and got a free key chain or a free magazine for joining the Sports Car Club of America.
We can't all be the NASCAR Indycar NHRA F1 world champion and make millions of dollars for driving a car, but for those minutes or seconds driving at autocross it probably feels kind of like that, and that's a valuable thing too.