You would expect a bigger drop.

Sold for a Voltaic Hat Protector Universal Translator(18 Keys)

http://backpack.tf/profiles/76561198142145406/#!/compare/1454313600/1455350400

Sold for a Blizzardy Storm Desert Marauder(19.5 Keys) and ~3.5 Keys in Sweets(23 Keys total)(B/O was 20)

http://backpack.tf/profiles/76561198276175323/#!/compare/1454832000/1454918400

http://backpack.tf/profiles/76561198087556034/#!/compare/1454659200/1454918400

http://www.tf2outpost.com/trade/27822495

Sold for an Australium Black Box(20 Keys at the time)(Item IDs match)

http://backpack.tf/profiles/76561198114965529/#!/compare/1455609600/1455696000

http://backpack.tf/item/4400607019

Sold for an Australium Stickybomb Launcher(17 Keys) and 3 Keys(20 Keys total)(Item IDs match)

http://backpack.tf/profiles/76561198134641756/#!/compare/1458777600/1458864000

Sold for a Disco Beat Down Dead Cone(15.5 Keys) and ~2.5 Keys in Sweets(18 Keys total)(B/O was 17)

http://backpack.tf/profiles/76561198143516878/#!/compare/1459209600/1459296000

http://www.tf2outpost.com/trade/28010134

Sales seem to fit, 17-20 Keys should be fine.

Additional Proof/Counter-Proof/Constructive Criticism/Feedback is always appreciated.