Proof:

http://www.tf2outpost.com/trade/8572033 (offered TS EA w/o wrap, worth about 5.5 keys)

http://www.tf2outpost.com/trade/8521123 (offered 4 keys + 3.66 + wrap; about 5 keys)

http://www.tf2outpost.com/trade/8609146 (Offered 5 keys w/o wrap; basically 4 keys. Selling for 4 Keys buyer provides wrap)

http://www.tf2outpost.com/trade/8419155 (Offered 3 keys + wrap by two people close to the bottom; offer was rejected.)

That's all the evidence I have right now; there's not a lot of Bounty Hat sellers on OP. Most if not all of these sellers rejected 3 keys + wrap, but someone had their B/O at 4, which is why I put the low end at 3.5. That concludes my first price suggestion; hopefully I did everything right :3