Using last month.

Sale 1:

Sold for $11.50 on marketplace.tf

https://marketplace.tf/items/1120;5;u3009

11.5/1.97 = ~5.84

X = 6

Sale 2:

Sold with ~1.5 keys in metal/items for Gains Stagger (7)

Seller: https://backpack.tf/profiles/76561198277345119#!/compare/1526083200/1526169600

Buyer: https://backpack.tf/profiles/76561198309261884#!/compare/1526083200/1526169600

X = 5.5

Sale 3:

Sold to a quickbuyer for 4 keys

Seller: https://backpack.tf/profiles/76561198075437543#!/compare/1526688000/1527379200

Buyer: https://backpack.tf/profiles/76561198048851745#!/compare/1526688000/1526774400

X = 4

Sale 4:

Sold for 1 key + ~3 keys in metal/items

Seller: https://backpack.tf/profiles/76561198294426922#!/compare/1526083200/1526169600

Buyer: https://backpack.tf/profiles/76561198277345119#!/compare/1526083200/1526169600

X = 4

Sellers:

~4.05 keys:

https://i.imgur.com/iN0npsG.png

https://backpack.tf/item/6388351107

https://backpack.tf/item/6848040927

~4.25 keys:

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

https://backpack.tf/item/2944696687

7 keys:

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

https://backpack.tf/item/6834003938

Sales at:

4 (2x)

5.5

6

5.5 and 6 are too high given the cheaper sellers. Excluding 5.5 and 6.

Going with 4 flat.