First suggestion was based off 1 sale, also the Secret Jeepcap involved in it was duped(not to mention the price outdated). There are 5 of these in existence, 2 are on outpost, both of which only have a single person history.

http://www.tf2outpost.com/trade/13465779

Sold for 2 buds + Black + Plain Bills + 18 keys = ~3.4 buds pure sale

http://www.tf2outpost.com/trade/13820148

Selling for 5 buds/$165(4.1 buds) (probably sold for that, he had multiple relists of this)

http://www.tf2outpost.com/trade/13256253

Sold for Roboactive Tricorne(~3) + Black Bills + 3 keys = ~3.5 buds - robo tricorne offered first

http://www.tf2outpost.com/trade/13722414 (current owner)

-best current offer I see is a Ghosts Hallmark(~3.9)