Just changing the range a bit from the previous suggestion.

Profestive Sugarcube's suggestion:

All those suggestions lead to this. And yet i'm still not finished.

Sale 1

http://backpack.tf/profiles/76561198056793444?time=1447574400&compare=1447488000 Bulk for Pro KS Aussie Scatter

Sale 2 OUTLIER

http://www.tf2outpost.com/trade/27282053Sold for OF Dragon + Stormy Head Warmer

B/O for DBD was 32

Warmer and OF had no trade, presuming offered

X = 20*0.9 + 25*0.9

X = 41

X = 32 (B/O)

OF http://backpack.tf/item/1188185274

DBD http://backpack.tf/item/2405009008

Warmer http://backpack.tf/item/1052726347

Mini for Dragon

http://backpack.tf/vote/id/566b26a7ba8d888a19a94cf5 20 keys, using 20

Mini for Warmer

Only tracable sale was with a Haunted Chicken Kiev for an MN Cap, which is about 9 keys, obviously low. http://www.tf2outpost.com/trade/27539584 Was offered 22 keys + Buds + Soldier's Killed here'

X = 22 + 2.5 + 0.5*0.9

X = 25

Guess I'll use 25

Sale 3

http://www.tf2outpost.com/trade/27098572 Sold for Smoking Lucky + 13.8 keys of sweets

Lucky had not trade, and was offered

X = 16.5*0.9 + 13.8*0.9

X = 27

Dragon http://backpack.tf/item/2640514091

Lucky + sweets http://backpack.tf/item/3389778454 http://backpack.tf/profiles/76561198069202582?time=1444546800&compare=1444287600

Mini for Lucky

http://backpack.tf/vote/id/566b3bccb98d88275de94d74 16.5 keys, using 16.5

Sale 4

http://www.tf2outpost.com/trade/27433134 Sold for DBD FMD

Dragon wanted to buy a Soldier Unusual, No overpay

B/O for FMD was 25

X = 18

FMD http://backpack.tf/item/2479647884

Dragon http://backpack.tf/item/3296916466

Mini for FMD

http://backpack.tf/vote/id/566b4cc7b98d88855ee94d4a#comment-566b4cc7b98d88855ee94d4b 17.5 keys, using 17.5

Sale 5

http://www.tf2outpost.com/trade/27363229 Sold for Duped Twatt Cold Killer

B/O for Dragon was 26

This Dragon is actually Duped, I'm being serious

X = 27

X = 26 (B/O)

Killer http://backpack.tf/item/2528089032

Dragon http://backpack.tf/item/3296916466

Mini for Killer

http://www.tf2outpost.com/trade/27019814 Sold for Logo Das Madden

B/o for Killer was 34

Madden had no trade, presuming offered

X = 29.5*0.9

X = 27

Madden http://backpack.tf/item/3321280975

Someone please remove that killstreak, the hat deserves better.

Killer http://backpack.tf/item/2138180160

Seller

http://www.tf2outpost.com/trade/27445459 30 keys, unsold after a month, proving the 32 sale to be an outlier

http://www.tf2outpost.com/trade/27475667 26 keys''

My evidence:

http://www.tf2outpost.com/trade/27275353 Sold for 23 pure after 2 weeks

http://imgur.com/zyf7cU3 selling in the classifieds for 23 keys

Not going to remove 27 since Sugarcube provided alot of evidence for that high end.