Sale 1:

Sold for $45 on Marketplace.tf

https://marketplace.tf/items/30545;5;u18

Buyer: http://backpack.tf/profiles/76561198251732346#!/compare/1488499200/1488585600

http://backpack.tf/item/3559644950

45/2.1 = ~21.4

X = 21

Sale 2:

Sold for Aces High Fed-Fightin’ Fedora (19) + <0.5 keys in items

No outpost

Seller: http://backpack.tf/profiles/76561198137326099#!/compare/1488844800/1488931200

Buyer: http://backpack.tf/profiles/76561198355458687#!/compare/1488844800/1488931200

X = 19

Sale 3:

Sold with ~3 keys in pure/items for Terror-Watt Caribbean Conqueror (19)

http://www.tf2outpost.com/trade/28820039 - B/O of 21

Seller: http://backpack.tf/profiles/76561198355458687#!/compare/1489017600/1489104000

Buyer: http://backpack.tf/profiles/76561198171624643#!/compare/1489017600/1489104000

X = 16

Sale 4:

Quicksold for 13 keys

http://www.tf2outpost.com/trade/28822699 - B/O of 20

Seller: http://backpack.tf/profiles/76561198171624643#!/compare/1489017600/1489104000

Buyer: http://backpack.tf/profiles/76561198227232564#!/compare/1489017600/1489104000

X = 13

Sale 5:

Quicksold for 12 keys

http://www.tf2outpost.com/trade/28817672 - No B/O

Seller: http://backpack.tf/profiles/76561198153095134#!/compare/1488758400/1488844800

Buyer: http://backpack.tf/profiles/76561198197251846#!/compare/1488758400/1488844800

X = 12

Sales at:

12

13

16

19

21

Sales at 12 and 13 quicksold. Excluding 12 and 13.

Excluding 16, looks like a bit of an outlier, and the range would look much neater without it.

Going with 19-21.