I thought this was going to be slow and punishing with the escalating hitpoints and cost inflation, but with the way the coin upgrade works, I quickly got to earning so many coins that the number didn't even fit on the screen, essentially breaking the game.
Never found out what the "magic power" and "damage timer" upgrades do, and "decrease cooldown" didn't seem to do anything either; I thought it would lower the auto-attack timer but it's at 0.00s now and nothing has changed.