I like the game, pace and everything, but money becomes really old really fast and becomes actually clutter in the screen in posterior levels. Maybe you could add like a really expensive upgrade, just so we get to spend it on something later on in the game? I don't really know. Maybe it could stop droping once you've bought everything? Maybe that was what Bowie was trying to tell us with "The Man Who Sold The World".
Anyway, congrats. :D