Nice game!
It reminds me of the ones i played on newgrounds or miniclips when i was a kid xD
I think the bullets are way to expensive. Per wave, the player is making around 200 or 250 gold and the bullets are 25 gold each so you can buy around 10 bullets. Now, each zombie takes at least 3 or 4 shots to kill with heavy bullets so you could only kill 3 of them at best.
I don't know if maybe i missed cheaper bullets, but, in case i didn't, maybe taking a look at the economy could improve the experience of the player!