The Miata is the most-raced car in the world. They're sublimely balanced and fun to drive, easy to work on, and economical to run. A ready-to-race DriveSkills SuperSpec car can be bought for around $15k. They run on pump gas and wear affordable Hankook tires that last several weekends. 

For more on the DNA of these rough-and-ready racers, see the Unofficial Supermiata Guide (pdf).

Platform	1st & 2nd Gen (1990-2005) Mazda Miata
Configuration	Front engine, rear drive
Power	140WHP (capped)
Comp Weight	2300lbs
Engine	1.8L NA 4-cylinder
Tires	Hankook Ventus RS4 - 225/45/15

Sample Lap Times	Track
1:59	Auto Club Speedway (Roval)
1:58	Buttonwillow (#13CW)
1:34	Willow Springs
1:55	Sonoma Raceway
2:03	Chuckwalla (CW)