Sale 1:

Sold with Panzer Pants (~1.5 at the time) for Tesla Cranial (Mini at 16.5 below)

Seller: https://backpack.tf/profiles/76561198177279603#!/compare/1547856000/1547942400

Buyer: https://backpack.tf/profiles/76561198251777504#!/compare/1547856000/1547942400

https://backpack.tf/item/6493644122

X = 15

Sale 2:

Sold for Abduction Head Full of Hot Air (13)

Seller: https://backpack.tf/profiles/76561198305494036#!/compare/1547769600/1547856000

Buyer: https://backpack.tf/profiles/76561198177279603#!/compare/1547769600/1547856000

https://backpack.tf/item/6493644122

X = 13

Other Sales:

Sold for Green Confetti Hat With No Name

https://backpack.tf/profiles/76561198251777504#!/compare/1548201600/1548288000

https://backpack.tf/profiles/76561198152590461#!/compare/1548201600/1548288000

https://backpack.tf/item/6493644122

Large outlier, excluding.

Tesla Cranial Mini:

Sale 1:

Sold for 17 keys

Seller: https://backpack.tf/profiles/76561198192602363#!/compare/1550361600/1550448000

Buyer: https://backpack.tf/profiles/76561198421240889#!/compare/1550188800/1550448000

https://backpack.tf/item/7504581215

X = 17

Sale 2:

Sold for 16 keys

Seller: https://backpack.tf/profiles/76561198035280371#!/compare/1547856000/1547942400

Buyer: https://backpack.tf/profiles/76561198202037046#!/compare/1547856000/1547942400

https://backpack.tf/item/5262949839

X = 16

Sale 3:

Sold for 16 keys + some ref

Seller: https://backpack.tf/profiles/76561198053811988#!/compare/1553212800/1553299200

Buyer: https://backpack.tf/profiles/76561198002757046#!/compare/1553126400/1553212800

https://backpack.tf/item/5240932886

X = 16

Sale 4:

Sold for 16 keys + 1 ref

Seller: https://backpack.tf/profiles/76561198859721638#!/compare/1553904000/1553990400

Buyer: https://backpack.tf/profiles/76561198129397184#!/compare/1553817600/1553990400

https://backpack.tf/item/5223808654

X = 16

Sellers:

https://i.imgur.com/68WibE8.png

https://backpack.tf/item/6973750870 - 15 keys

https://backpack.tf/item/6697921324 - 25 keys

Sales at:

13

15

Going with 13-15.