Check out this interesting series of articles from The Last Turn Clubhouse. It provides a comprehensive brakedown of the budget to run a car in the two major sports car racing series in the US - ALMS and Grand-am.

Few things I found interesting: The GT class is not much cheaper to run the the LMP classes in ALMS, and the Grand-am Daytona Prototypes are not significantly cheaper then ALMS prototypes, but still (according to the article) cheaper then the ALMS GT class!