Face Down + Hidden Mana

I'm not terribly bothered about what direction this goes, so long as it works well. But I'll say that if it completely (or largely) hides your mana then it's an additional boost to the card, and you should pay an appropriate extra amount for it.

Also, I am not a fan of randomness, but face down cards aren't random. They may lead to random decision making, but that's on the part of the player and not the cards.