Using last month

Sale 1:

Sold for 6 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198065409820#!/compare/1521763200/1521849600

Buyer: https://backpack.tf/profiles/76561198298909527#!/compare/1521331200/1522022400

X = 6

Sale 2:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198128655858#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198042132655#!/compare/1522454400/1523664000

X = 5

Sale 3:

Bulk for Power Surge Strontium Stove Pipe

Seller: https://backpack.tf/profiles/76561197969935601#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198131799675#!/compare/1521849600/1523145600

Sellers:

~5.55 keys:

https://i.imgur.com/C2GwuaS.png

https://backpack.tf/item/6075405855

6 keys:

https://i.imgur.com/8rmhko4.png

https://backpack.tf/item/5302308649

~7.5 keys:

http://www.tf2outpost.com/trade/29544473

8 keys:

https://i.imgur.com/CgaW0EP.png

https://backpack.tf/item/4398672591

9 keys:

http://www.tf2outpost.com/trade/29622946

10 keys:

https://i.imgur.com/E7HS4Fc.png

https://backpack.tf/item/2699332053

14 keys:

https://i.imgur.com/GBt4WOw.png

https://backpack.tf/item/4023238232

Sales at:

5

6

https://i.imgur.com/YkbbHGp.png - Buyer at 5 keys

https://i.imgur.com/Rg0iCO7.png - Buyers on the SCM for ~5 keys worth

Excluding 5.

Seller at 5.5, using that as the low-end.

Going with 5.5-6.