Use the following trick to earn $1 million in total over the course of Campaign mode. From the main menu, select "Campaign Mode", "Single Player", "Mission Select", then select the China mission. The first objective pays $30,000 and can be cycled quickly (about three minutes with co-op snipe). The checkpoint is back down the hill next to the hovercraft by the dock. Once you reach the checkpoint (which saves your money), quit and restart the mission. Repeat as many times as desired.