Using last month.

Sale 1:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198088488150#!/compare/1525305600/1525564800

Buyer: https://backpack.tf/profiles/76561198148495750#!/compare/1525564800/1525651200

X = 11

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525478400/1525564800

https://backpack.tf/item/6749713673 - Buyer can’t be traced, it sold to the bot, and then sold back to the bot before their backpack was scanned.

Bot.tf confirms: https://i.imgur.com/CBwMciI.png

X = 8

Sale 3:

Sold to a quickbuyer for 7 keys

Seller: https://backpack.tf/profiles/76561198148495750#!/compare/1525737600/1525824000

Buyer: https://backpack.tf/profiles/76561198048851745#!/compare/1525737600/1525824000

X = 7

Sale 4:

Sold to a bot for 5 keys + 10 ref

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1525564800/1525651200

https://backpack.tf/item/6749713673 - Same situation as Sale 2, except this time it’s the seller.

Bot.tf confirms: https://i.imgur.com/CBwMciI.png

X = 5.5

Sale 5:

Sold for 2 Strange Minimal Wear Coffin Nail Grenade Launchers (~6 each?) + ~1 key in items

No outpost

Seller: https://backpack.tf/profiles/76561198106600085#!/compare/1525219200/1525305600

Buyer: https://backpack.tf/profiles/76561198088488150#!/compare/1525219200/1525305600

Not comfortable using this sale here at all. Even if we do, using the value I’ve chosen, it’d be too high.

Sellers:

7 keys:

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

https://backpack.tf/item/6749713673

~7.3 keys:

https://i.imgur.com/PDe91eu.png

https://backpack.tf/item/6756382132

Offers:

http://www.tf2outpost.com/trade/29660694

Sales at:

5.5

7

8

11

11 is an outlier, excluding 11.

5.5 looks low, excluding 5.5.

Going with 7-8.