MQ-9 Reaper drones cost $30M a piece, so $50M for a brand new RQ-170 isn't a poor estimate, I would say.
Anyway, I read that it wasn't shot down, it was actually hacked by an Iranian "electronic warfare unit", who managed to land it with almost no damage. Seems plausible at least, considering these things would normally fly relatively high and to have shot it down, would have caused considerable damage, which is not visible on the released photographs.