6.5 seems far too low based on the offers and sales:

http://www.tf2outpost.com/trade/22807775 - offer of a Roboactive hat with no name (8~) + Strange Machina (1 bud)

Note: Guy declined it

Spartan sold for the HWNN and Machina as seen above, proof http://www.tf2outpost.com/trade/23068597

http://www.tf2outpost.com/trade/22044702 - Offer of a Peace and TF logo Rack (9~)

http://www.tf2outpost.com/trade/18719460 - Offer of a blizz yellow belt (2), c9 Pomm (4~) a bud and a s.f Knife(1) = 8 Buds

Note: The guy offering the belt and pomms above has offered to change the pomm to a max and keys

http://www.tf2outpost.com/trade/22390206 - Secret Snaggle (7.5-8~)

http://www.tf2outpost.com/trade/20024524 - Misty Skull Spartan offer (7~)

http://www.tf2outpost.com/trade/22807775 - DP Bonk Boy (4~) + Max (3.3) = 7.3 buds