I have to admit I didn't think about how much they should cost.
But a creature that is able to purify is wothier than 6 spells (maximum 60 cards,)
The idea for Behemoth was, thats its too easy when you own 2-3 Miracle (I have two, often there's no need to play them)
I wouldn't play cards like snowfall, but it's interesting if your opponent has one
The idea of healing poisoned creatures was similar: I play a
-based Rainbow-Deck with lots of poison, arsenic, plague, Parasite, toadfish,... i like poisonimg opponents because he hardly ever has a chance!!!
Like I said before there should be some more rares (really rare, not like nymphs - I got two in the first week, when they came into the game) and these are just some ideas.