I don't know, before the task system change, leveling new mercenaries was just insane and the first couple of mercenary additions was textbook power creep to feed sales. And the chances you'd get what you want were crazy.
But really the big think with hearthstone is BGs. There is a free mode that gets constant content and you can compete with no cost or a tiny cost.
I still think the RMAH in D3 was far worse than DI.