Using last month.

Sale 1:

Sold for $12 on marketplace.tf

https://marketplace.tf/items/30671;5;u3005

12/1.98 = ~6.06

X = 6

Sale 2:

Sold for 6 keys?

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198079474904#!/compare/1527811200/1532649600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/AOFneut.png

X = 6?

Sale 3:

Sold for 5 keys + small amount of metal

Seller: https://backpack.tf/profiles/76561198120566269#!/compare/1532304000/1532390400

Buyer: https://backpack.tf/profiles/76561198147269693#!/compare/1521244800/1532390400

X = 5

Sale 4:

Sold for 2 keys + ~2.5 keys in items

Seller: https://backpack.tf/profiles/76561198335687042#!/compare/1531785600/1531958400

Buyer: https://backpack.tf/profiles/76561198261565607#!/compare/1531872000/1531958400

Loch-n-load has a spell and sold for 1 key + 2 ref here https://backpack.tf/profiles/76561198378451439#!/compare/1526515200/1526601600

X = 4.5

Sale 5:

Sold to a bot for 3 keys + 12 ref?

Seller: https://backpack.tf/profiles/76561198307048774#!/compare/1531958400/1532304000

Buyer: https://backpack.tf/profiles/76561198382589541#!/compare/1532131200/1532217600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/AOFneut.png

X = 3.5?

Sale 6:

Sold to a bot for 3 keys + 14 ref?

Seller: https://backpack.tf/profiles/76561198391163452#!/compare/1531008000/1531440000

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1531440000/1532736000

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/AOFneut.png

X = 3.5?

Sale 7:

Bulk for pure

Seller: https://backpack.tf/profiles/76561198195734346#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198086070401#!/compare/1532304000/1532476800

Sellers:

8 keys:

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

https://backpack.tf/item/4612409037

9 keys:

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

https://backpack.tf/item/4604478886

https://backpack.tf/item/6172487731

10 keys:

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

https://backpack.tf/item/4534940696

Sales at:

4.5

5

6

And untraceable sales at 3.5 (2x) and 6.

Going with 4.5-6.