Using last month.

Sale 1:

Sold for 14 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1522886400/1522972800

Buyer: https://backpack.tf/profiles/76561198143710298#!/compare/1522713600/1522972800

Untraceable, but Bot.tf confirms: https://i.imgur.com/bETrBeB.png

X = 14

Sale 2:

Sold for $23.50 on Marketplace.tf

https://marketplace.tf/items/30570;5;u3010

23.50/1.97 = ~11.9

X = 12

Sellers:

11 keys:

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

https://backpack.tf/item/6140174694

12 keys:

https://i.imgur.com/Q7BlxRi.png

https://backpack.tf/item/3411246396

13 keys:

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

https://backpack.tf/item/4426790953

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

https://backpack.tf/item/3269460118

~13.5 keys:

https://i.imgur.com/irfz1nx.png

https://backpack.tf/item/6360802682

20 keys:

http://www.tf2outpost.com/trade/29558830

285 keys:

http://www.tf2outpost.com/trade/28672999

Sales at:

12

14

Given that the sale at 14 is nearly over a month old, is not entirely traceable, and every seller (on the classifieds) is currently asking for less than 14, I'd say it's safe to exclude 14 from the range.

Seller at 11, willing to let it mature, and use it as the low-end.

Going with 11-12.