Sometimes, suggestions are just pains.

Sale 1

[No OP Trade] Sold for 17 keys

X = 17

Keys https://gyazo.com/b1ab31911b9765a3e2158e6c372cd531 http://backpack.tf/profiles/76561198087962998?time=1448006400&compare=1447920000

Swag http://backpack.tf/item/1878735482

Sale 2

http://backpack.tf/profiles/76561198044333971?time=1447747200&compare=1447488000 Bulk for Hellfire Virus, can be used to price Virus

Sale 3

http://www.tf2outpost.com/trade/27133802 Sold for Gifted Blizz YWN + Spec KS S. Bargain

B/O for YWN was 18

YWN claimed it was "100% clean" when it was gifted and Duped, uh..

X = 18*0.9 + 1.3*0.9

X = 17

YWN http://backpack.tf/item/1584623659

Swag http://backpack.tf/item/1885345020

Mini for YWN

[No OP Trade] Sold for Twatt Fedora

X = 26*0.9

X = 23

Fedora http://backpack.tf/item/3193241918

YWN http://backpack.tf/item/1584623659

However considering the trade above had a B/O of 18, using 18