Face Down + Hidden Mana

Counting mana isn't going to do you much if any good. You won't know the mana he starts with. All you know is how much -additional- mana he could -possibly have- at a maximum and that's only if the face down is free. All you can do is add the amount he's generating and subract what he's playing - it isn't going to be worth it.

Once he plays a second face down or a face down effect you might as well give up trying.

As for the magic solution, it's pretty much impossible to make them all cost the same and keep them unique and varied as well as hving them different colors.