Really what I am try to say is its not balanced at the moment and introducing complicated reward algorithms might not be the best approach to fix a balance issue. Most games rebalance cards to fix balance issues. This 'simple game' is already extremely hard to explain to new comers.

Your point on price, higher level card would become more worth while but universal rewards are the same and all cards should be buffed proportionally so the only market shift I can see is combined cards not trading at as big of a discount.