9 months old price.

Only two on market at time of posting: http://m.imgur.com/x5XAbYQ

Using two recent sales I could trace.

Sale #1:

Sold for a Nuts n' Bolts Old Guadalajara:

http://www.tf2outpost.com/trade/26041075

History of the Head Warmer:

http://backpack.tf/item/1319548658

History of the Old Guadalajara:

http://backpack.tf/item/1977124280

The Old Guadalajara then sold with a buyout of 13 keys so valuing it at that since it has a year old price: http://www.tf2outpost.com/trade/26129549

No overpay is applied as the Guadalajara was offered

Total: 13 keys

Sale #2:

Sold for a Strange AWPer Hand: http://backpack.tf/item/3757387388 (It did not have professional killstreak then as the seller applied it afterwards)

History of the Head Warmer: http://backpack.tf/item/1319548658

Seller's backpack after: http://backpack.tf/profiles/76561198055494532?time=1432451012

Total: 11.5*0.9 = 10.35 keys

Rounding it off to 10 keys

Please tell me if I did anything wrong, thanks.