Ok, how was that economy set up?
I still believe it would be a better system than having relic trying to balance the game. As an example Kubels would be very expensive at the moment. It would disable abuse.
But what else would be expensive? All infantry would likely increase in price, since they're built more than other unit types. It's seldom that more than 2-4 snipers are built in a game, but what about Riflemen? Or Conscripts? Or Grenadiers?
I think balancing a 'market economy' RTS would have its own challenges. As someone else pointed out, some units are meant to be produced more than others. Also, this might discourage alternate strategies, since building more of something would be penalized. This would introduce a pretty stale meta, I think, where everyone just built 1-2 of the most effective units they could, then 1-2 of the next most effective, etc. I really dislike this idea.