I'm sure this range isn't too large

But, wow.

Sale 1

http://www.tf2outpost.com/trade/25690890 Sold for 16 keys pure

B/O for Stormer was 16 keys

X = 16

Barnstormer http://backpack.tf/item/3038285652

Keys http://gyazo.com/d0269b79b15849fb69638f01e1501ebd

Sale 2

http://www.tf2outpost.com/trade/24254691 Sold for 15 keys pure

X = 15

Keys http://gyazo.com/1fedbf498702515150cff4cd83ce61b1

Barnstormer http://backpack.tf/item/3038285652

Sale 3

http://www.tf2outpost.com/trade/24952544 Sold for P. Fetti Barnstormer + 8 keys

B/O for Barnstormer was 28 keys

X = 15*0.9 + 8

X = 22

Stuff http://gyazo.com/019e078f3ec0908390339a14ce3d1793

P. Fetti http://backpack.tf/item/3223184492

Stormy http://backpack.tf/item/2422548402

Sale 4

http://www.tf2outpost.com/trade/25777655 Sold for Searing Elfin

B/O for Stormer was 20

Elfin http://backpack.tf/item/3444935805

Stormer http://backpack.tf/item/2294854175

Using to price the Elfin.