Using last month

Sale 1:

Sold for 6 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198357046243#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198070559361#!/compare/1522972800/1523232000

Untraceable, but bot.tf confirms: https://i.imgur.com/0NnmRvr.png

X = 6.5

Sale 2:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198067663328#!/compare/1523059200/1523145600

Bot.tf confirms: https://i.imgur.com/0NnmRvr.png

X = 6

Sales 3 & 4:

Sold for $11.5 on marketplace.tf twice

https://marketplace.tf/items/1119;5;u3010

11.5/1.97 = ~5.84

X = 6 (2x)

Sale 5:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1523232000/1523318400

Buyer: https://backpack.tf/profiles/76561198141903490#!/compare/1519689600/1523318400

X = 5

Sale 6:

Sold for 4 keys + 3 ref

Seller: https://backpack.tf/profiles/76561198090904480#!/compare/1524614400/1524787200

Buyer: https://backpack.tf/profiles/76561198198332539#!/compare/1524700800/1524787200

X = 4

Sale 7:

Bulk for Burning Bonk Leadwear

Seller: https://backpack.tf/profiles/76561198067663328#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198135018939#!/compare/1522800000/1523145600

Sellers:

5 keys:

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

https://backpack.tf/item/6768727489

~5.6 keys:

https://i.imgur.com/I7pjOMe.png

https://backpack.tf/item/3256974607

~5.7 keys:

https://i.imgur.com/M4p6gM9.png

https://backpack.tf/item/2816366152

6 keys:

https://i.imgur.com/8j1YuJv.png

https://backpack.tf/item/5491192025

https://backpack.tf/item/2805478660

3rd one in the screenshot is one of the marketplace sellers that sold for $11.5. It’s likely that they sold both for $11.5.

13 keys:

https://i.imgur.com/mvmCY7L.png

https://backpack.tf/item/5593950195

Sales at:

4

5

6 (3x)

6.5

https://i.imgur.com/8oj89tj.png - Buyer at 4, excluding 4.

Sale at 6.5 looks like an outlier, given that there are 5 sellers for cheaper. On top of that, it’s untraceable. Excluding 6.5.

Going with 5-6.