Enable the "Master Code" then go to the car lot. You will find a red Acura Integra available for $0. You can sell it for $7,200 and can repeat this as many times as desired. You can also sell the other cars obtained when that code is enabled.

Enable the "Master code" then go to the career map and enter any event. Exit the event the first chance you get. After doing this, go into your garage. You will see that four new cars have been added to your collection: a Nissan 240 SX, a modern Pontiac GTO, a Chevy Cobalt SS, and a Dodge Viper SRT 10. You can keep any cars desired, but by selling all four of these newly added cars you can get $90,290 quickly. You can reset the game and repeat the process until you get the desired amount of money.

You can use the "Relock cars" code to sell your old cars again. Enable the "Master code" when you start the game and before you do any racing. Participate in a short race in career mode, like a 1/4 mile drag. When done, go to your garage. This will give back the old cars that you won and sold earlier. You can sell them again to get lots of money. You can repeat this as many times as desired, but must shut the game down and restart it each time to do this trick.

Use the following trick to get $91,500 in just a few minutes.

1. Enter Career mode. Select the "Secret Code" option at the menu.
2. Enable the "Master Code".
3. Go to any Race Day.
4. Immediately leave the Race Day.
5. Go to the garage. Notice that you now have four cars unlocked: the Pontiac GTO, Chevrolet Cobalt SS, Dodge Viper SRT10, and Nissan 240SX.
6. Sell them all. You can get $91,500 in total.
7. Quit the game.
8. Start the game again.
9. Repeat steps 1 through 4.
10. Those four cars will respawn in your garage.
11. Sell them all again.

Repeat all the steps as many times as desired to get money quickly in order to buy and upgrade your cars.