You can double your prize money by racing an opponent
and having less than half of the green that your opponent is betting.
Save the game before doing so -- if you lose, you will also lose
your car. If you concentrate on the expensive mods first (after
buying at least level 1 mod of everything), the other ones will
be easy to pay off once you hit class B/A, getting over $1,000
per race with this trick.