Using last month.

Sale 1:

Sold for $15 on marketplace.tf

https://marketplace.tf/items/30843;5;u3005

15/1.95 = ~7.69

X = 7.5

Sale 2:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1531699200/1531785600

Buyer: https://backpack.tf/profiles/76561198317608743#!/compare/1531699200/1531785600

X = 6

Sale 3:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198097002664#!/compare/1531785600/1531872000

Buyer: https://backpack.tf/profiles/76561198089802952#!/compare/1531785600/1531872000

X = 5

Sale 4:

Sold for Cloud 9 Polar Bear

No outpost

Seller: https://backpack.tf/profiles/76561198198992129#!/compare/1530748800/1532131200

Buyer: https://backpack.tf/profiles/76561198415625616#!/compare/1531958400/1532131200

Gonna be waaaaay too high to be used here, at least without a B/O.

Sale 5:

Bulk for Knifestorm Pardner’s Pompadour

Seller: https://backpack.tf/profiles/76561198148775366#!/compare/1532390400/1532476800

Buyer: https://backpack.tf/profiles/76561198418740133#!/compare/1532390400/1532476800

Sale 6:

Bulk for Vivid Starboard Crusader

Seller: https://backpack.tf/profiles/76561198098611646#!/compare/1531699200/1531785600

Buyer: https://backpack.tf/profiles/76561198232041512#!/compare/1531699200/1531785600

Sellers:

12 keys:

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

https://backpack.tf/item/6941221447

18 keys:

https://i.imgur.com/JMV5kex.png

https://backpack.tf/item/6402278211

Sales at:

5

6

7.5

Going with 5-7.5.