Using last month.

Sale 1:

Sold for $22.22 on marketplace.tf

https://marketplace.tf/items/30762;5;u3003

22.22/1.98 = ~11.2

X = 11

Sale 2:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1526083200/1526342400

Buyer: https://backpack.tf/profiles/76561198309361802#!/compare/1526342400/1526947200

Bot.tf confirms: https://i.imgur.com/zsXza5Z.png

X = 9

Sale 3:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198386441509#!/compare/1527292800/1527379200

Buyer: https://backpack.tf/profiles/76561198169109536#!/compare/1527292800/1527379200

Bot.tf confirms: https://i.imgur.com/zsXza5Z.png

X = 8

Sale 4:

Sold to a bot for 6 keys + 8.11 ref

Seller: https://backpack.tf/profiles/76561198114825374#!/compare/1525478400/1527552000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1525478400/1525564800

Bot.tf confirms: https://i.imgur.com/zsXza5Z.png

X = 6.5

Sale 5:

Bulk for Screaming Tiger Duelling Banjo

Seller: https://backpack.tf/profiles/76561198063054044#!/compare/1527379200/1527465600

Buyer: https://backpack.tf/profiles/76561197994697258#!/compare/1527379200/1527465600

Sellers:

10 keys:

https://i.imgur.com/MCWE50H.png

https://backpack.tf/item/6702224538

16 keys:

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

https://backpack.tf/item/6234997182

18 keys:

https://i.imgur.com/w3H8Eb3.png

https://backpack.tf/item/4955430890

https://backpack.tf/item/5905926664

19 keys:

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

https://i.imgur.com/rZQv1HI.png

https://backpack.tf/item/6593892293

https://backpack.tf/item/6269128782

24 keys:

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

https://backpack.tf/item/5141755499

Sales at:

6.5

8

9

11

https://i.imgur.com/Kv96TmS.png - Given that the market was quite a bit lower than 11, and the sale at 8 was originally a couple weeks unsold at 9 before selling, it should be sale to exclude 11 here.

6.5 looks low, excluding.

Going with 8-9.