Kinda high.

Sold for a Steaming Grimm Hatte(15 Keys)(B/O was 14)

http://backpack.tf/profiles/76561198080179568/#!/compare/1456560000/1456646400

http://www.tf2outpost.com/trade/27899882

Sold with ~4 Keys in Sweets(at the time) for an Australium Stickybomb Launcher(~18 Keys at the time)(14 Keys total)

http://backpack.tf/profiles/76561198119216883/#!/compare/1454140800/1454227200

http://backpack.tf/profiles/76561198044964602/#!/compare/1454140800/1454227200

Sold for a Strange Construction PDA(10 Keys) and 7 Keys(17 Keys total)

http://backpack.tf/profiles/76561198050745352/#!/compare/1458370800/1458457200

Sales seem to fit, 14-17 Keys should be fine.

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