I should be getting my homework done.

Sold for an Eerie Orbiting Fire Chieftain's Challenge(35 Keys), a Stormuy Storm Heer's Helmet(24 Keys) and ~4 Keys in Sweets(63 Keys total)

http://backpack.tf/profiles/76561198043543691#!/compare/1486857600/1487030400

http://backpack.tf/profiles/76561198090956917#!/compare/1486771200/1486857600

Sold for a Duped Knifestorm Industrial Festivizer(65 Keys)

http://backpack.tf/profiles/76561198177056801#!/compare/1481241600/1481328000

Sold for a Haunted Ghosts Desert Marauder(37 Keys, Outlier)

http://backpack.tf/profiles/76561198028959985#!/compare/1487289600/1487376000

Sold for a Time Warp Engineer's Cap(32 Keys) and ~31 Keys in Sweets(63 Keys total)

http://backpack.tf/profiles/76561198126204929#!/compare/1487548800/1487635200

Duped Knifestorm Industrial Festivizer mini suggestions

Sold for a Circling Peace Sign Chieftain's Challenge(~23 Keys) and ~44 Keys in Sweets(~67 Keys total)

http://backpack.tf/profiles/76561198075491803#!/compare/1481155200/1481241600

Sold for a Disco Beat Down Industrial Festivizer(~35 Keys) and ~34 Keys in Sweets(~69 Keys total)

http://backpack.tf/profiles/76561198002186909#!/compare/1482019200/1482105600

Both sales are estimates, but it's pretty clear that duped won't be below clean. Using the clean price of 65.

63-65.

Credits to Mindacos for the suggestion. He found the proof, I'm just posting it.