Sold mine for 2 Buds in cash fairly quickly. It was only posted on SourceOP when I sold it, nowhere else. And it had been listed for less than a day.

Proof: http://i.imgur.com/VlVg18E.png

There are two Misty Skull Mongolians in existence.

The other one was recently posted on Outpost, less than 30 minutes ago.

http://www.tf2outpost.com/trade/9316075

No offers yet. At least not when I made this price suggestion.

I know there isn't much evidence, but since only 2 of these exist, I don't exactly have much to work with beyond my own experience with the hat.