Big thanks to Jarool for all the help.

Sale 1 :

Sold on marketplace for 74.99$

https://backpack.tf/item/5195477803

https://marketplace.tf/items/30357;5;u107

So, 74.99/2.1= 35.7

Rounding to 36 keys

Sale 2 :

https://backpack.tf/item/5256917262

Seller : https://backpack.tf/profiles/76561198118325460#!/compare/1493164800/1493424000

Buyer : https://backpack.tf/profiles/76561198166102098#!/compare/1493164800/1493251200

Sold for an Isotope Killstreak Corsair Scattergun (Battle Scarred) ( 47.5 ) but taunts worth 5.8 keys and a skin worth 5.5 keys were added : 47.5 - 11.3 = 36.2 keys

Skin : http://steamcommunity.com/market/listings/440/Thunderbolt%20Sniper%20Rifle%20%28Minimal%20Wear%29

Sells for around 14$ (excluding qs) so rounding to 5.5 keys.

Sale 3 :

Sold for 28 keys

http://www.tf2outpost.com/trade/28836911 - B/O of 37

Seller: https://backpack.tf/profiles/76561198049072204#!/compare/1491091200/1491177600

Buyer: https://backpack.tf/profiles/76561198267335763#!/compare/1491091200/1491177600

X = 28

given that it's like 8 keys off the two sales at 36, and the sale is pretty old, i'd say that this one is an outlier

Pricing it at 36 keys.