Using only two recent sales.

Sale #1:

Sold for a Nuts n' Bolts Old Guadalajara:

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

History of the Guadalajara:

http://backpack.tf/item/1977124280

History of Warmer:

http://backpack.tf/item/1319548658

Guad was offered so no overpay applied

Guad has a year old price but the seller of the head warmer had a b/o of 13 keys when he sold it:

http://www.tf2outpost.com/trade/26129549

Total: 13 keys

Sale #2:

Sold for a Strange AWPer Hand: http://backpack.tf/item/3757387388 (It did not have professional killstreak when it was sold to the seller, he applied it himself)

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

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

Total: 11.5*0.9 = ~10 keys