[Proof below] There are 3 of this hat in existence, one is in the original owner's backpack, the other was used to make this suggestion and the other I sadly couldn't find.

Sales

http://www.tf2outpost.com/trade/11601612 - For Overclocked Le Party Phantom (~ 2.5 buds)

http://www.tf2outpost.com/trade/11791014 - For ????

http://www.tf2outpost.com/trade/12990289 - For Sulphurous Shooter's Sola Topi (~ 1.9 buds)

Active Trade

http://www.tf2outpost.com/trade/11456863 - Unboxer

All offers are either declined or retracted:

Bubbles Shooter's Sola Topi (~ 12 keys)

20 keys

2 buds + 16 keys + #12 Iron Mask = ~ 3.1 buds

Oops missed out a small declined offer from the active trade. Bubbling Brown Bomber (~ 2.2 buds) + Headshot Kills S.Part = ~ 2.3 buds. Fits within my suggested range but was still a declined offer as it was hidden by the trade owner. I also believe that Trade 2 of the sales was sold for Haunted Ghosts Fuka: http://www.tf2outpost.com/trade/12772555 (2 buds when accounting for overpay).