Using last month.

Sale 1:

Sold for 11 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198083997589#!/compare/1526515200/1526601600

Buyer: https://backpack.tf/profiles/76561198147239913#!/compare/1526256000/1526601600

X = 11

Sale 2:

Sold for 11 keys

Seller: https://backpack.tf/profiles/76561198177822454#!/compare/1527120000/1527206400

Buyer: https://backpack.tf/profiles/76561198052264884#!/compare/1527120000/1527206400

X = 11

Sale 3:

Sold for $19.98 on marketplace.tf

https://marketplace.tf/items/1168;5;u3004

https://backpack.tf/item/5416952033

19.98/1.97 = ~10.1

X = 10

Sale 4:

Sold for 9 keys + 32 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1527120000/1527206400

Buyer: https://backpack.tf/profiles/76561198063510973#!/compare/1527120000/1527206400

Bot.tf confirms: https://i.imgur.com/4aqZSwp.png

X = 10

Sale 5:

Sold to a bot for 6 keys + 8 ref

Seller: https://backpack.tf/profiles/76561198204099043#!/compare/1525564800/1525737600

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

Bot.tf confirms: https://i.imgur.com/4aqZSwp.png

X = 6.5

Sale 6:

Bulk for Lantern Tyrantium

Seller: https://backpack.tf/profiles/76561198147239913#!/compare/1526601600/1526774400

Buyer: https://backpack.tf/profiles/76561198158206236#!/compare/1526688000/1526774400

Sale 7:

Bulk

Seller: https://backpack.tf/profiles/76561198158206236#!/compare/1527120000/1527206400

Buyer: https://backpack.tf/profiles/76561198177822454#!/compare/1527120000/1527206400

Sales at:

6.5

10 (2x)

11 (2x)

No current sellers.

6.5 seems low and uncommon, excluding.

Going with 10-11.