To unlock all cars, press UP, LEFT [two times], RIGHT, DOWN, RIGHT, L1, SQUARE at the title screen. (Note: This cheat also awards players with additional money!)

For 10 additional cars, simultaneously press Square, X, Triangle and Start.

To unlock additional races, press DOWN [two times], LEFT, RIGHT, UP, LEFT, CIRCLE, R2.

In the 2nd class of races, go to the left until you come to the pink and silver Saleen Mustang. Buy it for 30,000$, then you should be able to sell it for 55,000$. Keep this up and you can have all the money that you want.

(Note: it will not work with any other car.)