After all...

Sold for a Terror-Watt Milkman(19 Keys)

http://backpack.tf/profiles/76561198082520148/#!/compare/1455782400/1457510400

Sold for a Bubbling Frenchman's Beret(13.5 Keys) and ~6.5 Keys in Sweets(20 Keys total)

http://backpack.tf/profiles/76561198082381645/#!/compare/1457683200/1457856000

Sold for a Steaming Brain Bucket(15 Keys) and 3 Keys(18 Keys total)

http://backpack.tf/profiles/76561198131636781/#!/compare/1456214400/1456646400

Sold with ~0.5 Keys in Sweets for an Orbiting Fire Chieftain's Challenge(19.5 Keys)(19 Keys total)

http://backpack.tf/profiles/76561198151725535/#!/compare/1457078400/1457164800

Sales seem to fit, 18-20 Keys should be fine.

Additional Proof/Counter-Proof/Constructive Criticism/Feedback is always appreciated.