how much does it cost to race a sprint car ? i.e. cost per event etc.?

It really depends on where you're going to race, and what class of sprint car. The track's, or the club's rules for their sprint car class can make racing outrageously expensive, or relatively affordable. World of Outlaw sprint cars, new, with a top of the food chain engine are in the 75k to 90k range. Of course that's not significant, compared to the cost of operation. For example a nightly tire budget, to race competitively runs in excess of $500 per night. An engine refresh on an aluminum 410, after half a season, can cost 7k or more.

On the other hand, there are some great sprint car groups around the country that are on the way to making this wonderful class of racing the best game in town. The best set of rules to model is a club in the Dallas TX area run by a man named Smiley Sitton. I think its called Super Sprints of Texas (SST). A guy can race a sprinter there on a budget slightly higher than a street stock due to his clever engine and tire rules. The neat thing is that the cars are light and fast and the fans really can't tell the difference between Smiley's cars and any other sprint car class. Also, google a man named French Grimes, and his budget sprint car formula "race saver". Actually, I think Sitton patterned his series from Grimes rules.

Best of luck.