It isn't going to be cheap unless it is just a demo derby type of car. If you actually want to go fast around a road race course then it is going to cost money. Starting with a rusted shell of a car that you get for free might seem like the cheap way to go but I have my doubts. First step would be to pull one of those hulks out of the yard and have it blasted just to see what you have. You might find that you can't even work with the metal on those cars. Better figure that out first before you make too many plans.