Street Legal Cheats

First you race everyone in the game and even the cars that you usually aren't able to race you can race just by honking the horn at them.
Then you have invincibility on your car so it won't get crushed or even the paint job scratched.

Note: This procedure involves changing game files;
create a backup copy of the files before proceeding. Open the
game folder and find the save folder that you want to get more
money for. Open it and copy any PlayerCar file
desired and rename it to a number you do not have. If you have
a file such as PlayerCar.
(for example, PlayerCar1.2) and it belongs to the car you want
to copy (for example, PlayerCar3.4 belongs to PlayerCar3), change
is the number before the decimal point. Then, start your game,
load your save file, and go to the car lot. You will find your
copied car there. Choose it and sell it. Repeat this process for
as much money as needed.
Alternately, copy the "PlayerCar1" file and rename it
to "PlayerCar2". If you need more money, you can keep
going up in the car numbers such at "PlayerCar3", "PlayerCar4",
etc. Keep increasing the numbers, but make sure to keep the same
name as "PlayerCar".
Sometimes when you race a car, it does not move. If so, race it
again repeatedly to get plenty of money.
Trainer: +1 by franky0529
Trainer: +1 by Tone Kravanja

Drive up to a pedestrian and press [Horn] to honk your horn. A menu screen will appear and give you choices of finding a shop, a race, and just forgetting about talking. Choose to find a race and a car will appear on the map. Go to that area. When you get near to the car, honk the horn. The driver will say "You honk your horn at me punk?". Press [Enter] and go to the bottom right hand corner. Set what you want to race for (prestige or money).