resuggesting; 1 of 1 in existence!

Sale 1:

Sold for Darkblaze Killer's Exclusive (550*0.9) = 495 keys (rounding up to 500 keys)

http://www.tf2outpost.com/trade/25758694

http://i.imgur.com/os81TJ1.png

Sale 2: (invalid sale, outdated and duped hats)

Sold for 2 Duped Lantern Soldier's Stashes

http://www.tf2outpost.com/trade/24971358

http://i.imgur.com/Xogf0HQ.png

Sale 3: (credits to Neutron [SC-Λ] for clearing this sale up!)

Sold for Sunbeams Dread Hiding Hood = 445 keys (no overpay, was offered on; rounding up to 450 keys)

http://www.tf2outpost.com/trade/25720711

http://www.tf2outpost.com/item/440,3650063597,1095,5 → http://www.tf2outpost.com/item/440,3705450567,30003,5

Notes:

Hat in question: http://backpack.tf/item/3163093866

http://www.tf2outpost.com/trade/25739194 - B/O of 600 keys

http://www.tf2outpost.com/item/440,3705450567,30003,5 - The Cloudy Moon Gibus did not have spΛce :/'s name in the history because he traded it for the Sunbeams Dread Hiding Hood almost right after he got it from Neutron [SC-Λ].

Conclusion:

Two valid sales at ~500 and ~450 keys, along with a recent B/O of 600 keys. Taking a final range of 450-500 keys.

Feel free to support/counter my suggestion!