7 month old price!

Sale 1:

Sold for 18 keys

http://www.tf2outpost.com/trade/26990314

http://backpack.tf/profiles/76561198031371895?time=1441782000 (September 9th)

http://backpack.tf/profiles/76561198031371895?time=1441868400 (September 10th)

http://backpack.tf/profiles/76561198031371895?time=1441868400&compare=1441782000

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

Sale 2:

Involved in a bulk deal

http://backpack.tf/profiles/76561198117050098?time=1439794800 (August 17th)

http://backpack.tf/profiles/76561198117050098?time=1440226800 (August 22nd)

http://backpack.tf/profiles/76561198117050098?time=1440226800&compare=1439794800

Sale 3:

Sold for Stormy Storm Vintage Tyrolean

http://www.tf2outpost.com/trade/27025142

http://backpack.tf/profiles/76561198130359264?time=1442127600 (September 13th)

http://backpack.tf/profiles/76561198130359264?time=1442214000 (September 14th)

http://backpack.tf/profiles/76561198130359264?time=1442127600&compare=1442214000

Sale 4:

Sold for Stormy Storm Milkman = <15 keys

http://www.tf2outpost.com/trade/27024797

http://backpack.tf/profiles/76561198080533870?time=1442127600 (September 13th)

http://backpack.tf/profiles/76561198080533870?time=1442214000 (September 14th)

http://backpack.tf/profiles/76561198080533870?time=1442127600&compare=1442214000

Sale 5:

Sold for Steaming Brain Bucket + 2 keys + Sweets (5.45*0.9) = >20 keys (B/O caps it at 20 keys)

http://www.tf2outpost.com/trade/26624626

http://backpack.tf/profiles/76561198142855504?time=1438671600 (August 4th)

http://backpack.tf/profiles/76561198142855504?time=1438758000 (August 5th)

http://backpack.tf/profiles/76561198142855504?time=1438758000&compare=1438671600

Sale 6:

Sold for Kill-a-Watt Brim-Full Of Bullets

http://backpack.tf/profiles/76561198101057741?time=1441695600 (September 8th)

http://backpack.tf/profiles/76561198101057741?time=1441782000 (September 9th)

http://backpack.tf/profiles/76561198101057741?time=1441695600&compare=1441782000

Sale 7:

Involved in a bulk deal

http://www.tf2outpost.com/trade/26998750

http://backpack.tf/profiles/76561198035694566?time=1441954800 (September 11th)

http://backpack.tf/profiles/76561198035694566?time=1442041200 (September 12th)

http://backpack.tf/profiles/76561198035694566?time=1441954800&compare=1442041200

Sale 8:

Sold for Bubbling Katyusha + Sweets (9.9*0.9) = >20 keys (B/O caps it at 20 keys)

http://www.tf2outpost.com/trade/27017474

http://backpack.tf/profiles/76561198048292656?time=1442041200 (September 12th)

http://backpack.tf/profiles/76561198048292656?time=1442127600 (September 13th)

http://backpack.tf/profiles/76561198048292656?time=1442041200&compare=1442127600

Notes:

Most other sales are either unidentifiable, involved in bulk deals or have outdated hats.

http://www.tf2outpost.com/trade/27025142 - B/O of 19 keys

http://www.tf2outpost.com/trade/26990314 - B/O of 20 keys

http://www.tf2outpost.com/trade/27017474 - B/O of 20 keys

http://www.tf2outpost.com/trade/26624626 - B/O of 20 keys

Conclusion:

Three valid sales at 18, 20, and 20 keys. Taking a final range of 18-20 keys.

Feel free to support/counter my suggestion!