Buy a new cheap car like a Chevy Aveo and upgrade it as much as you can.

If you have a small amount of money left go to engine and then go to air filter and press A to go in. When you are in press start and select "reset to default" and then press A and you will exit.

Then press your Xbox button and then press X to sign out.

Then when you sign back in, go to the menu and you will have the same amount of money that you had before you upgraded! Also your car will still have the upgrades!

Then you can sell them for even more money!