Using last month

Sales 1 & 2:

Sold for $13 twice on Marketplace.tf

https://marketplace.tf/items/30614;5;u3008

13/1.97 = ~6.6

13/1.95 = ~6.66666666

X = 6.5 (2x)

Sale 3:

Sold for 5 keys + 8 ref

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1521504000/1521590400

Buyer: https://backpack.tf/profiles/76561198274046400#!/compare/1521504000/1521590400

X = 5.5

Sale 4:

Bot.tf sale at 5 keys

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

Can’t find the items history

X = 5

Sellers:

5 keys:

https://i.imgur.com/w4hRzaK.png

https://backpack.tf/item/6711644279

~5.2 keys:

http://www.tf2outpost.com/trade/29616525

https://backpack.tf/item/4444098589

~5.55 keys:

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

https://backpack.tf/item/6480348882

7 keys:

https://i.imgur.com/G6wkH08.png

https://backpack.tf/item/4445295317

Sales at:

5

5.5

6.5 (2x)

3 sellers at the 5-5.5 key range, 6.5 looks like an outlier. Excluding 6.5

Going with 5-5.5.