Using last month.

Sale 1:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198086076383#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198263342776#!/compare/1523145600/1523232000

X = 8

Sale 2:

Sold for $14 on Marketplace.tf

https://marketplace.tf/items/30671;5;u3010

14/1.97 = ~7.11

X = 7

Sale 3:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198062249974#!/compare/1525219200/1525305600

Buyer: https://backpack.tf/profiles/76561198126985704#!/compare/1525219200/1525305600

X = 5

Sale 4:

Bulk for Steaming Federal Casemaker and kazotsky kick

Seller: https://backpack.tf/profiles/76561198126985704#!/compare/1525219200/1525305600

Buyer: https://backpack.tf/profiles/76561198271971982#!/compare/1525132800/1525305600

Sellers:

30 keys:

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

https://backpack.tf/item/6071067278

31 keys:

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

https://backpack.tf/item/5009505111

Sales at:

5

7

8

5 seems rather low/outlier-ish, and the buyer may have been quickbuying. Excluding 5.

Going with 7-8.