~1-2 month old price!

Sale 1:

Sold for 9 keys

http://www.tf2outpost.com/trade/27947022

http://backpack.tf/profiles/76561198187233450/#!/compare/1458716400/1458802800

http://backpack.tf/profiles/76561198031371895/#!/compare/1458716400/1458802800

Sale 2:

Sold for 9 keys + sweets = >11 (B/O caps it at 11 keys)

http://www.tf2outpost.com/trade/27900114

http://backpack.tf/profiles/76561198103207318/#!/compare/1457251200/1457510400

Sale 3: (credits to ♥Prof. Sugarcube™♥)

Sold for 11 keys

http://www.tf2outpost.com/trade/27786907

http://backpack.tf/profiles/76561198028693738/#!/compare/1454572800/1454659200

Sale 4: (credits to ♥Prof. Sugarcube™♥; somewhat old but can be used as a support-sale)

Sold for 9 keys

http://www.tf2outpost.com/trade/27722903

http://backpack.tf/profiles/76561198075045316/#!/compare/1453536000/1453708800

Notes:

Considering that there are now two sales at 9 keys as well as myself getting next to no offers on the hat for months, 9 keys should be included.

Conclusion:

Four valid sales at 9, 11, 11 and 9 keys. Taking a final range of 9-11 keys.

Feel free to support/counter my suggestion!