I think the cost should be increased, considering that most permanents that you'd target cost more. Maybe a cost increase of 1 or 2?
I thought this:
4/3 
Steal. Opponent lose permanent and you gain it.
3/2 
Deflagration. Opponent lose permanent.
2 
+ 1 
+1 turn wait / 1 + 1 +1 turn wait = 3/2 +1 turn wait : You gain same opponent's permanent. Seems fair.
4
+ 1 card = [5] -> Steal Permanent (aka Copy and Destroy Permanent)
3
+ 1 card = [4] -> Destroy Permanent
3
+ Duo + 1 turn wait + 1 card = [6] -> Copy non item permanent?
Fair? No it is not fair.
Nor is it an apt comparision. Compare it rather to hard casting another of the permanent.
Ex: Unstable Gas + Mirror Tower vs 2x Unstable Gas
6
+2
+2
+ 1
+2 turns of wait + 2 card +3 (Quad) = [18]
12
+2
+2 turns of wait + 2 card +1 (Duo) = [19]
Graveyard + Crystal Tower vs 2x Graveyard
5
+2
+ 1
+1 turn of wait + 2 cards + 2(Trio) = [13]
10
+ 2 cards = [12]
But then the issue of 'copy' comes in. Parallel Universe costs 7/6. Why should this cost so much less? 7/6 means that it doesn't make sense to copy a smaller creature, most of the time. Phase dragon was nerfed because it could be copied, in element. While this isn't prevalent in Aether today as its creatures are either immaterial or too small to pay off, permanents do not have the same limitation.
Consider that you could gain, say, an extra 6 mindgates, sanctuaries, shards of Gratitude/void- I'm sure that 6 of these in Zen could make for quite a powerful deal.
Creatures can be much more valuable than non item permanents. Crimson Dragon >> Unstable Gas
