Bringing this down once more:

http://www.tf2outpost.com/trade/14315247 (2 weeks):

Best offer was 12 keys

Offer was rejected, but also retracted as buyer isn't buying anymore: http://www.tf2outpost.com/trade/14628071

http://www.tf2outpost.com/trade/14611286 (4 days):

willing to sell for 10 keys - http://i.imgur.com/0wyAYyK.png

http://www.tf2outpost.com/trade/14284124:

Sold for a strange PDA (7 keys), Strange Scatter (2.66 ref) ~ 7.44 keys (taking this as an outliner)

Vuvezela + 6 keys sold for steaming VRH (19 keys) ~ making the vuvezela worth around 13 keys.

http://www.tf2outpost.com/trade/13702557 - vuvuzela

http://www.tf2outpost.com/trade/14284401 - headset

http://backpack.tf/id/76561198021082168?time=1378018800 - before

http://backpack.tf/id/76561198021082168?time=1378623600 - after