Using last month.

Sale 1:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198195731583#!/compare/1534032000/1534118400

Buyer: https://backpack.tf/profiles/76561198000889832#!/compare/1533859200/1534118400

https://backpack.tf/item/5285244109

X = 14

Sales 2 & 3:

Sold for $24.99 and $16.20 on marketplace.tf

https://marketplace.tf/items/1111;5;u3005

https://backpack.tf/item/3580976692

Couldn’t find the other one

http://manicbot.me/keyprice/?at=2018-07-27 - 16.2/1.95 = ~8.31

http://manicbot.me/keyprice/?at=2018-08-18 - 24.99/1.89 = ~13.22

X = 13

X = 8.5

Sale 4:

Sold for Circling Heart Coldfront Commander

Banned on outpost, no B/O there.

Seller: https://backpack.tf/profiles/76561198197739413#!/compare/1533513600/1533600000

Buyer: https://backpack.tf/profiles/76561198104119175#!/compare/1533513600/1533772800

https://backpack.tf/item/3274786018

Way too high, excluding.

Sellers:

https://i.imgur.com/laBSXxh.png

https://i.imgur.com/haYDfHI.png

https://backpack.tf/item/4190324296 - 15 keys

https://backpack.tf/item/4374513263 - 16 keys

https://backpack.tf/item/5868876868 - 22 keys

https://backpack.tf/item/4320081557 - 22 keys

https://backpack.tf/item/3645277001 - 34 keys

https://backpack.tf/item/6099320282 - 35 keys

https://backpack.tf/item/5619671066 - 45 keys

https://backpack.tf/item/4790506804 - 50 keys

Sales at:

8.5

13

14

Excluding 8.5, looks like an outlier, most likely went to a marketplace.tf buy order, and is rather close to buyers.

Going with 13-14.