A year outdated? For a commonly traded hat?...

Sale 1

http://www.tf2outpost.com/trade/26538891 Sold with 3 keys for DBD Grimm

Grimm had no trade, presuming offered

X + 3 = 26*0.9

X = 20

Grimm http://backpack.tf/item/2291463577

Authority http://backpack.tf/item/1711697435

Keys https://gyazo.com/7d5581535135920e6663d473be0f8f31 http://backpack.tf/profiles/76561198062145654?time=1437548400&compare=1437375600

Mini for Grimm

http://backpack.tf/vote/id/55e571bfdea9e96f238b45f0 26 keys, using 26

Sale 2

http://www.tf2outpost.com/trade/26045640 Sold for MN Fez

B/O for Authority was 22

B/O for Fez was 22

X = 18.5*0.9

X = 17

Fez http://backpack.tf/item/2242723335

Authority http://backpack.tf/item/1711697435

Mini for Fez

http://backpack.tf/vote/id/55e589e8b98d882c258b45d9 18.5 keys, using 18.5

Sale 3

http://www.tf2outpost.com/trade/26806880 Sold for Stormy Bubble Pipe

B/O for Authority was 24

Pipe had no trade, presuming offered

X = 20*0.9

X = 18

Pipe http://backpack.tf/item/929661361

Authority http://backpack.tf/item/1558946420

Sale 4

[No OP Trade] Sold for 24 keys pure

X = 24

Keys http://i.imgur.com/lcAdGqD.png

Authority http://backpack.tf/item/1515888614

Sellers

http://www.tf2outpost.com/trade/26846795 23 keys