Using last month.

Sale 1:

Sold for 13 keys

Seller: https://backpack.tf/profiles/76561198162282453#!/compare/1526169600/1526256000

Buyer: https://backpack.tf/profiles/76561198403404710#!/compare/1526169600/1526342400

X = 13

Sale 2:

Sold for 12 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525046400/1525132800

Buyer: https://backpack.tf/profiles/76561198162282453#!/compare/1525046400/1525132800

Bot.tf confirms: https://i.imgur.com/sNjEOC1.png

X = 12

Sale 3:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1525651200/1525737600

Buyer: https://backpack.tf/profiles/76561198276373160#!/compare/1525651200/1525737600

Bot.tf confirms: https://i.imgur.com/sNjEOC1.png

X = 11

Sale 4:

Sold to a bot for 7 keys + 10.11 ref

Seller: https://backpack.tf/profiles/76561198306942136#!/compare/1525478400/1525564800

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

Bot.tf confirms: https://i.imgur.com/sNjEOC1.png

X = 7.5

Sale :

Sold to a quickbuyer for 7.5 keys

Seller: https://backpack.tf/profiles/76561198276373160#!/compare/1526428800/1526515200

Buyer: https://backpack.tf/profiles/76561198125791160#!/compare/1526342400/1526428800

X = 7.5

Sellers:

33 keys:

https://i.imgur.com/KC7Hd3J.png

https://backpack.tf/item/6592261872

Sales at:

7.5 (2x)

11

12

13

7.5 looks kinda low, excluding.

Going with 11-13.