Sale 1:

Sold for Max’s (~18) + Legendary Lid (~2)

No outpost

Seller: https://backpack.tf/profiles/76561198182281603#!/compare/1499731200/1499817600

Buyer: https://backpack.tf/profiles/76561198324137456#!/compare/1499731200/1499904000

X = 20

Sale 2:

Sold for 15 keys

http://www.tf2outpost.com/trade/29193457 - B/O of 18, was offered 16.

Seller: https://backpack.tf/profiles/76561198088533549#!/compare/1496707200/1498608000

Buyer: https://backpack.tf/profiles/76561198058368145#!/compare/1496275200/1497571200

X = 15

Sale 3:

Bulk for DBD Impaler

Seller: https://backpack.tf/profiles/76561198108340555#!/compare/1499385600/1499558400

Buyer: https://backpack.tf/profiles/76561198182281603#!/compare/1499472000/1499558400

Marketplace Sales:

Sale 4:

Sold for $43.99 on Marketplace.tf

https://marketplace.tf/items/539;5;u60

https://backpack.tf/item/1929978206

43.99/2.02 = ~21.8

X = 22

Sale 5:

Sold for $37 on Marketplace.tf

https://marketplace.tf/items/539;5;u60

https://backpack.tf/item/2743341331

37/2.1 = ~17.6

X = 18

Sales at:

15

18

20

22

No current sellers.

15 looks pretty low, especially considering the seller had an offer of 16 keys. Excluding 16.

Going with 18-22.