You must be connected to Xbox Live to do this.
Go to your customize ride then to the far left of that menu which should be Xbox live market place.
Click this and Download the Supreme Car package.
Then go to the car lot "buy" as many Aston Martin DB9 as you can then save your game.
Go to your Xbox dashboard then to System then into your memory option then into games. Click on your need for speed game and Delete the supreme package.
Then go back to your career and sell your DB9s for easy money
Repeat as necessary.
Reason for removing the package is because the cars value is 0 when you buy and 0 when you sell unless you delete the supreme package.
Just found this today very handy.