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, press B to 
return to the first screen, then press Y 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.