0 is the right amount (upped). You get 6, you "wasted" 6 cards and can play with 1 card in your hand, while your opponent didn't lose any and plays with the card he draws for 3 turns, then he "gets his full hand back". It's even UP to make them cost 0, imo. It's not like you can target cards in the hand and destroy them while blocked, which is one of the main aims of congeal.