Sale 1:

Sold for $23.99 on Marketplace.tf

https://marketplace.tf/items/181;5;u35

Could not identify the history.

23.99/2.07 = ~11.6

X = 12

Sale 2:

Sold for 10 keys

http://www.tf2outpost.com/trade/29174149 - B/O of 10

Seller: https://backpack.tf/profiles/76561198054004552#!/compare/1494720000/1494806400

Buyer: https://backpack.tf/profiles/76561198310661143#!/compare/1494720000/1494806400

x = 10

Sale 3:

Sold for $21 on Marketplace.tf

https://marketplace.tf/items/181;5;u35

https://backpack.tf/item/1295879735

21/2.04 = ~10.3

X = 10

Sale 4:

Sold for 10 keys

http://www.tf2outpost.com/trade/28903876 - B/O of 10

Seller: https://backpack.tf/profiles/76561198107520045#!/compare/1494720000/1494806400

Buyer: https://backpack.tf/profiles/76561198191140763#!/compare/1494720000/1494806400

X = 10

Sale 5:

Sold for 9 keys + <0.5 keys in metal

No outpost

Seller: https://backpack.tf/profiles/76561197963500354#!/compare/1501372800/1501459200

Buyer: https://backpack.tf/profiles/76561198201974823#!/compare/1501286400/1501459200

X = 9

Sale 6:

Quicksold for 8 keys

http://www.tf2outpost.com/trade/29185069 - B/O of 11

Seller: https://backpack.tf/profiles/76561198191140763#!/compare/1494979200/1495065600

Buyer: https://backpack.tf/profiles/76561198146428703#!/compare/1495065600/1495238400

X = 8

Sale 7:

Bulk for Frostbite KE

Seller: https://backpack.tf/profiles/76561198049834485#!/compare/1494374400/1494460800

Buyer: https://backpack.tf/profiles/76561198054004552#!/compare/1494201600/1494460800

Sellers:

11 keys:

http://i.imgur.com/daSTAPU.png

Sales at:

8

9

10 (3x)

12

12 looks like a bit of an outlier, especially with some of those sales at 10 taking quite a while to sell. Excluding 12.

8 quicksold, and there are buyers higher than 8. Excluding 8.

Going with 9-10.