this is it. i did once. easy but too much work
Infinite money
This trick requires some starting money, a private lobby on Xbox Live, and an online friend.
1. Buy the 1969 Chevrolet Camaro Z28.
2. Buy all engines for that Camaro.
3. Buy every part for every engine, including all aspiration parts.
4. Exit to the main menu to save the engines and parts that were purchased.
5. Enter the engine purchase menu, and sell every engine. Note: Do not leave the engine menu.
6. Press the Xbox Guide button, and join your friend's private lobby.
7. Leave the lobby, then quickly press B to exit to the title screen, with the "Press A to Start" message.
8. Enter the engine purchase menu. You will still own all the previously purchased engines and parts and have the money you got from selling the engines.
9. Repeat steps 5 through 8 as many times as desired to get an unlimited amount of money.