Interesting concept but unbalanced. 3rd tier was too hard and required a grind, after that everything was a breeze until very least level. Or maybe I just purchased and extremely powerful hero at this point I dunno. The very large gray thing vaguely resembling biplane from front seems to be very strong.
Last level was normally extremely hard and would require great grind, until you notice that at this point game gives you a free hero that is immune to zombies and kills all enemies by itself making it easy.
Only way to make money at decant speed was to kill boss over and over. The rewards for completing missions need to be bigger to not make players grind on killing the weak but much money rewarding boss.
More description on what each hero does before buying him would also be nice. Further this could be improved by adding strategy if you could chose your starting placement of heroes on start of mission, for instance place sniper behind fire-thrower.