RSX: mildly affordable
NSX: More expensive than RSX
Miata: pretty much contends because its light or if it is a Mazdaspeed
Sentra: a good choice
Golf: easily affordable and good hp for the money
Lancer:reg lancer? (evos and eights are expensive)
A Very Affordable List of easily tunable cars
Acura Integra
Nissian 240sx
Honda prelude
Nissian 300zx
Ford Focus
Honda del sol
Ford Mustang
Honda civic
volkswagon golf
jetta