Credit to Slocumruls for proof. 1 of 1 hat, unboxable for another year. Re-suggesting as polar said 4 pure should be the low end.

Sale #1:

Sold for 4 pure, but had a B/O of 3.5 pure (polar said it should be the 4 pure here used for the low end):

Before: http://www.tf2outpost.com/trade/23636419

After: http://www.tf2outpost.com/trade/23637953

Sale #2:

Sold to Slocumruls for 5 pure (He's a collector so he's not going to sell it):

Trade: http://www.tf2outpost.com/trade/23637953

Trade history: http://imgur.com/gv0FWKF and http://imgur.com/Br3LeTZ

Outpost history: http://www.tf2outpost.com/item/440,3174347717,30041,5

bp.tf history: http://backpack.tf/item/3174003892

Additional proof/credits: http://i.imgur.com/DNX6FWW.png

Last suggestion: http://backpack.tf/vote/id/549695d9ba8d88c51e8b47d1#comment-54970866b98d88fa458b46c0