A good way to earn more money per second is to shoot really high up and let them fall on your target, then upgrade how many cannonballs you can have out at once, because you get way more bullets on the screen due to the longer time it takes to hit the target.
Just thought I'd share a helpful tip for this awesome game.
SEQUEL HAS BEEN RELEASED! http://ninjakiwi.com/Games/Arcade/Bloons-Super-Monkey-2.html
Great improvements made to the game. Takes a lot longer to complete though, not a one-day game.
See my reply to aghloe.