Using last month.

Sale 1:

Sold for $30 on marketplace.tf

https://marketplace.tf/items/30845;5;u3001

https://backpack.tf/item/6252757542 - I don’t think Kevin was the one that sold it.

30/1.98 = ~15.2

X = 15

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198442764620#!/compare/1524700800/1524787200

Buyer: https://backpack.tf/profiles/76561198044157063#!/compare/1524700800/1524873600

Bot.tf confirms: https://i.imgur.com/m24ZLQN.png

X = 11

Sale 3:

Sold for 9 keys + 20 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198058281379#!/compare/1525392000/1525478400

Untraceable, but bot.tf confirms: https://i.imgur.com/m24ZLQN.png

X = 9.5

Sale 4:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198044157063#!/compare/1524873600/1525046400

Buyer: https://backpack.tf/profiles/76561198026640869#!/compare/1524441600/1525046400

X = 9

Sale 5:

Bulk for Circling Heart Field Practice

Seller: https://backpack.tf/profiles/76561198121555078#!/compare/1525737600/1525824000

Buyer: https://backpack.tf/profiles/76561198093926010#!/compare/1525737600/1525824000

Sale 6:

Bulk for Strange Burning Field Practice

Seller: https://backpack.tf/profiles/76561198093926010#!/compare/1525824000/1525996800

Buyer: https://backpack.tf/profiles/76561198065409820#!/compare/1525910400/1525996800

Sellers:

9 keys:

https://i.imgur.com/9iVTRma.png

https://backpack.tf/item/6253417769

~9.7 keys:

https://i.imgur.com/ntt76wp.png

https://backpack.tf/item/6512285274

~10.8 keys:

https://i.imgur.com/X33sIWo.png

https://backpack.tf/item/6757433760

11 keys:

https://i.imgur.com/geWnH24.png

https://backpack.tf/item/6523147840

Offers:

http://www.tf2outpost.com/trade/29509043

Sales at:

9

9.5

11

15

15 is a clear outlier, excluding 15.

Going with 9-11.