Full credits to Mindacos

Sale 1:

Sold for an Orbiting Fire Private Eye (Suggestion at 14.5)

https://backpack.tf/profiles/76561198126985704#!/compare/1508112000/1508198400

It was Spec KS at the time of the trade, however the Spec KS was apparently removed later. Valuing it at regular price.

X = 15

Sale 2:

Sold for a Stormy Storm Connoisseour's Cap(12.5 Keys, round to 13)

https://backpack.tf/profiles/76561198044735367#!/compare/1508371200/1508457600

https://backpack.tf/profiles/76561198214364347#!/compare/1508371200/1508457600

X = 13

Sale 3:

Sold for 12 Keys

https://backpack.tf/profiles/76561198108148076#!/compare/1507852800/1508112000

X = 12

Sale 4:

Quicksold for 11 Keys

https://backpack.tf/profiles/76561198214364347#!/compare/1508371200/1508457600

https://backpack.tf/profiles/76561198078081841#!/compare/1506211200/1508371200

X = 11

Sellers:

22 keys:

http://www.tf2outpost.com/trade/29452267 - 1 day

Sales at:

11

12

13

15

11 quicksold, excluding 11.

Going with 12-15.