Sale 1:

Sold for 10 keys

http://www.tf2outpost.com/trade/29378518 - B/O of 12

Seller: https://backpack.tf/profiles/76561198061489334#!/compare/1506124800/1506211200

Buyer: https://backpack.tf/profiles/76561198228933180#!/compare/1506124800/1506211200

X = 10

Sale 2:

Sold for 8 keys + > 0.5 keys in items

No outpost

Seller: https://backpack.tf/profiles/76561198119581864#!/compare/1500681600/1500768000

Buyer: https://backpack.tf/profiles/76561198126979379#!/compare/1500681600/1500768000

X = 9

Sale 3:

Sold for 9 keys + 15 ref

http://www.tf2outpost.com/trade/29291156 - B/O of 10

Seller: https://backpack.tf/profiles/76561198126979379#!/compare/1500768000/1500854400

Buyer: https://backpack.tf/profiles/76561198154109244#!/compare/1500768000/1500854400

X = 9

Sale 4:

Sold for 9 keys to a bot

Seller: https://backpack.tf/profiles/76561198262288177#!/compare/1500854400/1500940800

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1500854400/1500940800

X = 9

Sale 5:

Bulk for Green Energy Herald’s Helm

Seller: https://backpack.tf/profiles/76561198154109244#!/compare/1500854400/1500940800

Buyer: https://backpack.tf/profiles/76561198262288177#!/compare/1500854400/1500940800

Sellers:

10 keys:

https://i.imgur.com/5CGGF3h.png - 2 months

Sales at:

9 (3x)

10

Sales at 9 are far too common to be excluded, even with the buyer at 9.

Going with 9-10.