Sale 1:

Sold for $37 on Marketplace.tf

https://marketplace.tf/items/390;5;u7

Buyer: http://backpack.tf/profiles/76561198010188541#!/compare/1492646400/1493942400

http://backpack.tf/item/343914803

37/2.1 = ~17.6

X = 18

Sale 2:

Sold for Australium Grenade Launcher (~14) + Burstchester (~3) in less than a day

http://www.tf2outpost.com/trade/29006089 - B/O of 20

Seller: http://backpack.tf/profiles/76561198029493014#!/compare/1494115200/1494201600

Buyer: http://backpack.tf/profiles/76561198167939691#!/compare/1494115200/1494201600

X = 17

Sale 3:

Bulk for Sulphurous Cold Killer

Seller: http://backpack.tf/profiles/76561198018719204#!/compare/1489968000/1490054400

Buyer: http://backpack.tf/profiles/76561198048297608#!/compare/1489968000/1490054400

Sale :

Bulk for Chiroptera Venenata Scotch Bonnet

Seller: http://backpack.tf/profiles/76561198048297608#!/compare/1490140800/1490227200

Buyer: http://backpack.tf/profiles/76561198083030915#!/compare/1490140800/1490227200

Sellers:

~15.7 keys:

http://i.imgur.com/TUUAEiS.png - 3 days, definitely re-listed from a higher price:

http://backpack.tf/item/343914803 - Has owned for around 3 weeks

http://backpack.tf/u/76561198304138933

22 keys:

http://i.imgur.com/7j3GbE6.png - 7 months

http://backpack.tf/u/76561198086948995

Offers:

http://www.tf2outpost.com/trade/28989394 - 1 month

http://backpack.tf/u/76561197961847363

Sales at:

17

18

Seller at 15.7, rounding to 16

Going with 16-18.