What is the Forza Motorsport 2 money cheat

Its not a cheat; just a glitch. Here's how to do it:

To get a lot of money without racing, buy a 2005 Chevrolet Corvette C6. Go to the buy upgrades section and then the engine upgrades section. Buy all of the upgrades (every level in every section), but do not buy the upgraded engine. After you have purchased all of the upgrades, buy the other engine for the car, then install the stock engine. Now sell the non-stock engine. If you do this correctly you will be able to buy the other engine for about 32,000 CR and sell it for around 124,000. At this point you can repeat the buy engine/equip stock engine/sell engine routine to make a lot of money really fast. There is no limit to how many times you can do this.

I have not tried this as apparently it can get you banned from Xbox Live.

This glitch will not get you banned from Xbox Live or Forza Motorsport matchmaking. The game has had this bug patched out, however it is still accessible by deleting all software updates from the console. To stop people from rolling back their data, the developers have implemented a cap on how much money can be imported from older versions of Forza Motorsport 2 in to newer versions. The player can get around this limit by building up a large amount of money with the older version, filling up their garage with multiple models of the same expensive vehicle, patching their game, and selling all of the cars they bought beforehand through either the ingame store or the online auction house.