We have an "Infinite Credits" cheat here on IGN Cheats, but if you want to collect money rather easily (and honestly), try undertaking one of the five major fetchquests in the game (all UNC): Asari Writings, Locate Signs of Battle, Prothean Data Discs, Turian Insignias, and Valuable Minerals. These missions combined, if you were to collect everything above and beyond the limit provided to complete the side quest, are worth in excess of 5,000,000 credits, more money than you'd really know how to spend in the entire game.