Get as many of the treasure chests as you can in the first level and kill all of the enemies you can. Leave one alive, however. When you get to the end of the level, go find the enemy again and die. When you replay the level, you'll still have all of your cash and the chests and enemies will respawn. Do this and by the middle of you're third life, you'll have over 1000 coins. Make sure not to spend any money in the buildings.