Successfully complete any mission and earn as little money as possible. Save the game, then replay that mission again. Earn as much money as possible and create a save point immediately before escaping from the level. Then, escape from the level and the mission review screens will appear displaying your new account balance. Any additional money earned from playing the mission the second time will be added to your account balance. Next, go back to the first screen, then select to replay the mission. A message warning that your progress will be lost will appear. Ignore this message, as your save points and account balance will not be reset. When the mission begins again, load the save point created immediately before escaping from the level. Escape from the level and the additional money will be added to your account balance again. Repeat this process as much as desired to get an unlimited amount of money.