The problem with using ads to try to reward players for using the ads is that you want the players to play long enough for that to pay out. If you make the grind too steep, as is the case with this game (it should never take more than a few levels to get an upgrade, and enemies that take more to kill should drop more of the game's currency, not the same, or there's little reason to bother killing them) then folks will *not* want to play your game at all.