I want to buy Some cars like the Ford GT40, Toyota GT One Race car, RSX Type-S, but when I try to buy them the game won't let me. I notice there's no price on the car. So how do I unlock it so I can buy them?
You can't buy them. You have to win them from doing series races. Checkout the prize car FAQ to see which races you need to do to win the car you want.
The reason why they do let you buy those cars is because a) they aren't in production and b) they're concept cars. Last time I checked, the RSX Type S wasn't out in 2000.
