Not really interested in dying over and over again because of the lack of cash in the early stages and don't mind the monotony of MMMORPG farming? 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 opening stage movie, press SQUARE (or TRIANGLE).

Just as an amendment to this technique (which works really well). If you set your partner to Aggressive Advance mode (by hitting up twice), he will run forward and clear the way for you all the way up to the location where you have to snipe at the bridge. Then, as you're sniping the bridge, he'll run back down towards the checkpoint that must be hit before you restart the game to do it all over again. It's not necessarily a faster way to do this, but it does require a lot less walking.