Great idea, great gameplay. Sounds and graphics are great. This game would be a total 10, however... the upgrade system absolutely destroyed it. It simply takes too long to buy the upgrades, and their improvements are just not worth it.
Why is this mistake so severe? Simple. The player simply can't enjoy the full potential of the game if he can't progress. And let's face it, few people have the time and enthusiasm to spend hours on the same task for no noticable reward.
I, for instance, consider myself a good medal hunter here on NG, still I couldn't take it.
Now the good news is that this may be a very easy fix for you. Either make upgrades cheaper or put some more coins out there, otherwise people wont be able to enjoy this (which is a shame, because, like I said, this game has everything to be a front page here).
Keep it up man!