I'd be fine with $40. That's what the Crusader expansion was. Would anyone really NOT get this if it was $40? $20? 30$ 40$ -- In the grand scheme of how much time you will be putting into Challenge rifts, and a brand new class, how can $40 be too much money compared to $20?
You are comparing an expansion with a new character, act , adventure mode and other features(i think) to a character pack. There is no way they would charge that much for a single class and 2 new areas.