Cheats > Game > Cheat: 50653

Easy money
Select the game file that you want to double the money in. With another controller, load the same game file. There should be two identical characters. Start a level and when it begins, attack "yourself" and collect the the money that is dropped. You actually do not have to spend money again; save your game before every purchase, then repeat the duplicate character loading process to get all your money back.