First of all, well done for making it on Steam Greenlight! I'll be interested to know how your journey goes!
Secondly, there are some pretty affordable artists around the place. There is an ongoing thread somewhere (I beleive RandomExile posted it up) with sites. I personally have had a good experience using elance before, and it didn't cost an arm and a leg.
As with the other guys above, the graphics are your first impression, so if they are underwhelming, people won't be interested. IMHO your graphics are nice and simple, but they are still dev art which will risk giving your game an amateurish look.