I don't mind the cost. Consider: Fate Egg has summoning sickness, SoSe you have to then pay to play the cards. SoSe is about card advantage, this is about quanta advantage. Which may be an argument for lowering the cost, since it should cost less than the average card
I lean towards letting it play Nymphs, consider Fate Egg. Don't know about non collectibles. Ash and Malignant would be meh. Singularity would be cruel
Does this include pillars?
Average cost of a card: 4.1; 4.3 if you don't count any 0 cost cards. 3.5 if you count pillars
So I change my opinion. this should cost 3