In my opinion, racing for money doesn't really work all that well unless you're Alberto Contador.

The most I'd win at a mountain bike race over the summer was a t-shirt or cap or something. They don't give the junior raffle booze tickets.

As for jobs, I'm sure you can start wrenching at an LBS over the summer. You're 17, so the legal age thing isn't an issue. Do you have any bike maintenance experience? We have a bike club at school and I volunteered as the club mechanic. Several members of the club are successful local high school racers, and it's funny the day before race day when my garage is lined with their speed machines. Fulfilling.