10 month old price!

Sale 1:

Sold for Searing Plasma Safe'n'Sound (25.5*0.9) + Sweets (~1) = 22.95 + 1 = 23.95 (rounding to 24 keys)

http://www.tf2outpost.com/trade/25442875

http://backpack.tf/profiles/76561198088777482?time=1427958000 - before

http://backpack.tf/profiles/76561198088777482?time=1428130800 - after

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

Sale 2:

Sold for 17 keys

[no outpost trade]

http://backpack.tf/profiles/76561198071167440?time=1427612400 - before

http://backpack.tf/profiles/76561198071167440?time=1427698800 - after

http://i.imgur.com/11eRJV2.png

Sale 3: (invalid sale, was used in another suggestion)

Sold with Stormy 13th Hour Fed-Fightin' Fedora for Frostbite Backwards Ballcap

http://backpack.tf/profiles/76561198065879305?time=1426748400 - before

http://backpack.tf/profiles/76561198065879305?time=1426834800 - after

http://i.imgur.com/RsN3dzq.png

Sale 4: (outdated hat, but should still be within suggested range)

Sold for Blizzardy Storm Reggaelator = ~21 keys

http://www.tf2outpost.com/trade/25503400

http://backpack.tf/profiles/76561197969724767?time=1424592000 - before

http://backpack.tf/profiles/76561197969724767?time=1426575600 - after

http://i.imgur.com/juK3fWG.png

Conclusion:

Two valid sales at 17 keys and 24 keys, along with a support-sale at ~21 keys. Taking a final range of 17-24 keys.

Feel free to support/counter my suggestion!