The “going rate” for Legendaries in Hearthstone is an average of 1 in 20 packs. I’m about to watch more friends open packs and play some games to earn the 10 free packs you get. And I do have 8-9+ of the tri-class rares, now that I'm looking back, which does seem weird. Update 2: Alright, I opened 60 more packs, this time going 3 for 60, exactly average. Looking deeper into the tri-class situation, in 165 packs I have 17 Kabal Couriers, 15 Lotus agents and 15 Grimstreet Informants, all tri-class rares.

Source: Forbes December 01, 2016 17:29 UTC