Try a new campaign in China. The first objective pays $30,000 and can be cycled quickly (about 3 minutes with co-op snipe). The checkpoint is back down the hill next to the hovercraft. Once you get the checkpoint (saves your money), quit and restart the stage. Tedious but effective. You only wish you could make $10,000 per minute.

To skip the pre-rendered cut-scene (and speed this up more), press BLUE (X).