I have played 1106 Crucible games... Mostly in Control/IB. If my memory serves correct, I have received 2 Exotic Weapons (Gjallarhorn and 4th Horseman) in this time. So for me, the drop rate appears to be 1/550 games... so due to randomness, this is probably 1/500 (1 in 500).
Whats been your drop rate?
EDIT #1: Strange Distribution so far..."Law of Small Numbers" I guess...
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I'd estimate it's right around 1 in 400, and I have a huge sample size of games to go off. Lost count of exactly how many exotics I've had though :/