Using last month.

Sales 1 & 2:

Sold for $19.49 and $17.99 on Marketplace.tf

https://marketplace.tf/items/1113;5;u3007

19.49/1.97 = ~9.89

17.99/1.97 = ~9

X = 10

X = 9

Sale 3:

Sold for Steaming Outdoorsman

http://www.tf2outpost.com/trade/29641169 - B/O of 8

Seller: https://backpack.tf/profiles/76561198093765664#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198120940117#!/compare/1525392000/1525478400

Easily caps.

X = 8 (Capped by B/O)

Sale 4:

Sold for 7 keys + 25 ref

Seller: https://backpack.tf/profiles/76561198442764620#!/compare/1524182400/1524355200

Buyer: https://backpack.tf/profiles/76561198340621517#!/compare/1523923200/1524268800

Untraceable (last 3 keys would’ve gone towards the Most Wanted), but bot.tf confirms: https://i.imgur.com/tQM7dMU.png

X = 8

Sale 5:

Sold for 7 keys + ~0.5 keys in items

Seller: https://backpack.tf/profiles/76561198306916766#!/compare/1524268800/1524528000

Buyer: https://backpack.tf/profiles/76561198310631632#!/compare/1524355200/1524614400

X = 7.5

Sale 6:

Quicksold for 5 keys + 16 ref

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1524787200/1524873600

Buyer: https://backpack.tf/profiles/76561198809718054#!/compare/1524787200/1524873600

X = 5.5

Sale 7:

Sold for Showstopper Spent Well Spirits

Seller: https://backpack.tf/profiles/76561198241622178#!/compare/1523577600/1523664000

Buyer: https://backpack.tf/profiles/76561198092619740#!/compare/1523577600/1523664000

Can be used on the Spirits. Might end up being too high there and too low here though.

Sellers:

16 keys:

http://www.tf2outpost.com/trade/29485310

18 keys:

https://i.imgur.com/WBAW2WJ.png

https://backpack.tf/item/5210591490

70 keys:

https://i.imgur.com/E51j4GX.png

https://backpack.tf/item/4218915748

Sales at:

5.5

7.5

8 (2x)

9

10

5.5 quicksold, excluding 5.5.

Going with 7.5-10.