Using last month.

Sales 1-7:

Sold for $29.49, $28.49, $27, $22, $20, $18.5, and $18 on marketplace.tf.

https://marketplace.tf/items/30672;5;u3004

29.49/1.96 = ~15

28.49/1.95 = ~14.6

27/1.96 = ~13.8

22/1.9 = ~11.6

20/1.99 = ~10.1

18.5/1.96 = ~9.4

18/1.98 = ~9.1

X = 15 (2x)

X = 14

X = 12

X = 10

X = 9.5

X = 9

Sale 8:

Sold for 12 keys?

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1530835200/1530921600

Buyer: https://backpack.tf/profiles/76561198108853200#!/compare/1530835200/1530921600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/blq4TDR.png

X = 12

Sale 9:

Sold to a bot for 7 keys + 13 ref

Seller: https://backpack.tf/profiles/76561198259589902#!/compare/1530576000/1530748800

Buyer: https://backpack.tf/profiles/76561198335791307#!/compare/1530662400/1530748800

Bot.tf confirms: https://i.imgur.com/blq4TDR.png

X = 7.5

Sale 10:

Bulk for Beams Woodsman

Seller: https://backpack.tf/profiles/76561198133360797#!/compare/1530921600/1531094400

Buyer: https://backpack.tf/profiles/76561198232041512#!/compare/1531008000/1531094400

Sale 11:

Bulk for Burning Bonk Leadwear

Seller: https://backpack.tf/profiles/76561198113302325#!/compare/1529884800/1529971200

Buyer: https://backpack.tf/profiles/76561198061489334#!/compare/1529884800/1529971200

Sellers:

12 keys:

https://i.imgur.com/E0v6AM4.png

https://backpack.tf/item/5899164112

13 keys:

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

https://backpack.tf/item/4631377893

15 keys:

https://i.imgur.com/c27PFIk.png

https://backpack.tf/item/4773072811

23 keys:

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

https://backpack.tf/item/6432920206

31 keys:

https://i.imgur.com/0UXoUGF.png

https://backpack.tf/item/4559231854

42 keys:

https://i.imgur.com/VDG7BRj.png

https://backpack.tf/item/6803182819

Sales at:

7.5

9

9.5

10

12

14

15 (2x)

And an untraceable sale at 12.

The sellers at 12 and 13 have remained unsold for a while, excluding anything higher.

7.5 looks like an outlier, excluding 7.5

Going with 9-12.