Answer
On the twin turbo supra - about $6k or higher mazda rx7 - about $3k or higher (for turbo models are around $5.5k or higher)and m3 coupes - bare minimum is around $9k or $10k(unless you want a wrecked one)if you like ebay thats the lowest place to go if not then search the web for used cars like: carmax, wheels masazine, and used car dealerships
