I recently sold it for 11 Keys pure..

It' s one in existence according to backpack.tf

There is a second sale as you can see below at 12 Keys pure..

(If you check the history, the Unusual Horseless Headless Horsemann's Headtaker traded for the Non-Craftable Professional Killstreak Ambassador, so the rest 12 Keys traded for the Purple Confetti Cadaver's Cranium)

Proof here: http://imgur.com/a/6MSpJ