Really old price. Over a year old.

http://www.tf2outpost.com/trade/17801015

Offered on a hearts trilby + 2 buds. Appears to have sold for Tribly + 29 keys

B: http://backpack.tf/id/76561198062936443?time=1391760000

A: http://backpack.tf/id/76561198062936443?time=1391846400

Sales Price: 3.9 + 1.5 = 5.4/0.9 = 6 buds

http://www.tf2outpost.com/trade/17145827

Burning Flames Medic Otolaryngologist's Mirror + 2 buds

Mirror priced 3-3.2 June 28 when buds were ~22.5 keys

Adjusted price = 22.5/19.5 * 3.1 = 3.5 buds

(Reality check, http://www.tf2outpost.com/trade/17516880 claims a 3 bud offer, B/Os are at 4)

Sale Price: ~5-5.5 buds

http://www.tf2outpost.com/trade/17266718

Quick sold for 4.5 pure

Current Trades: (In general, B/Os are 8 for the FYI)

http://www.tf2outpost.com/trade/17863600

-PE marauder (4-5)

http://www.tf2outpost.com/trade/17634454

-oc tyrant + 50 keys (maaaaaybe 4.5?)

-secret salty dog (~6)

- Tentative 5, but was rejected anyway

So it's a bit tough to nail down, but it looks like 5.5-6 is the right price that going to be fair to buyers and sellers.

let me know if you have any add'l info!