This is really good and original, well as far as it can zombie killing spree games can get anyways lol.
the concept of health cost is a really good unique concept but is annoying when you last really after your last health up(since you need to kill so many zombies) and you cant keep up and end up takin 10 with bairly any health. what might make it better if you had like a timer on the health cost, so say after a 20seconds, it drops, more after 40, and so on

i think health shouldn't be such a big change after u buy one health pack so expensive couldn't even buy the second one and there aren't alot of healing packs tat drop kinda gets boring maybe there could be a round refersher after so many kills or new sets of guns

but somethings:

HUGE health intervals: It's hard to get health, and the fact that it practically triples when you buy something doesn't really help

Slow money gain: Kinda annoying, since the health is expensive and the items are too

there may be more problems, but I don't know

i loved this game but i think that the arena was two small... should have made it longer.

Holy Shit! Wow! Crazy! Insane! My new favorite game of all time!

!!!!!!PROBLEM!!!!!!!
i got to 800 kills and zombies stopped coming =(
i might tray again later, your rock man!