Using last month.

Sale 1:

Sold for 17 keys

Seller: https://backpack.tf/profiles/76561198095022965#!/compare/1525132800/1525219200

Buyer: https://backpack.tf/profiles/76561198104574882#!/compare/1525219200/1526342400

X = 17

Sale 2:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198091666617#!/compare/1525132800/1525219200

Buyer: https://backpack.tf/profiles/76561198053514673#!/compare/1525132800/1525219200

X = 8

Sale 3:

Sold for $15 on Marketplace.tf

https://marketplace.tf/items/30842;5;u3010

15/1.97 = ~7.61

X = 7.5

Sale 4:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198121830870#!/compare/1525392000/1525478400

Buyer: https://backpack.tf/profiles/76561198164041055#!/compare/1525392000/1525478400

X = 7

Sale 5:

Sold to a bot for 6 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198053514673#!/compare/1526860800/1526947200

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1526860800/1526947200\

Bot.tf confirms: https://i.imgur.com/nRA6vQa.png

X = 6.5

Sale 6:

Bulk for Burning Western

Seller: https://backpack.tf/profiles/76561198081907770#!/compare/1524960000/1525046400

Buyer: https://backpack.tf/profiles/76561198095022965#!/compare/1524960000/1525046400

Sellers:

8 keys:

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

https://backpack.tf/item/6287949267

https://backpack.tf/item/6255751571

~8.1 keys:

http://www.tf2outpost.com/trade/29654195

12 keys:

http://www.tf2outpost.com/trade/29640601

Sales at:

6.5

7

7.5

8

17

17 is a large outlier, excluding.

Ignoring that buyer at 8 because I highly doubt they're actually buying.

Going with 6.5-8.