So since there was no price and I just acquired mine, I decided I would make a suggestion.

Only two of them. Only one that was sold on Outpost by Hugh | trade.tf #music. He was asking for 55 keys.

http://www.tf2outpost.com/trade/27416168 Got it for 49,5 keys.

Proof: http://i.imgur.com/PdluQkf.png

One of the previous owner Yahiamice sold it for an Harvest Moon Berliner's Bucket Helm (60 keys)+ sweets.

http://www.tf2outpost.com/trade/26968640

Seems pretty reasonable to suggest a solid 50 keys.

Cheers!