Looks like the game is pretty broken right now. Upgrade the first stand to level 200 and the manager to level 200, and you can see the money multiply in what seems like a very unintended way.
Got a bunch of buzz saws, turrets and launchers, let it sit while I was asleep and got my combo over 69 million and got over $1 billion. And I used that money way too quickly, which is the only thing wrong with this game. The cost of items goes up exponentially, while your ability to get money doesn't.
http://FantasticContraption.com/?designId=4973684
Simplest way to beat the wall.
By the way, whether he hits the wall or not, War Wrecker's design is simpler.
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 "Epic Combo!"){kind=icon}
 "Obey the Game"){kind=icon}
 "Dolphin Olympics 2"){kind=icon}
 "The Last Stand 2"){kind=icon}
 "Fantastic Contraption"){kind=icon}