Sale 1:

Sold for 17 keys in a day

http://www.tf2outpost.com/trade/28454894 - B/O of 17

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1472601600/1472688000

Buyer: http://backpack.tf/profiles/76561198026591178#!/compare/1472515200/1472688000

http://puu.sh/qX7ik/27648331a1.png

http://puu.sh/qX7jx/399cbc470d.jpg

X = 17

Sale 2:

Sold for Cloud 9 Defiant Spartan

http://www.tf2outpost.com/trade/28438087 - B/O of 16

Seller: http://backpack.tf/profiles/76561198092471230#!/compare/1471910400/1471996800

Buyer: http://backpack.tf/profiles/76561198077605729#!/compare/1471651200/1471996800

Spartan is outdated, however, there are buyers at 15 keys, so this should easily cap at the B/O

X = 16 (Capped by B/O)

Sale 3:

Sold with 1 key in items for Blizzardy Panama (16)

No outpost

Seller: http://backpack.tf/profiles/76561198114671331#!/compare/1471651200/1471910400

Buyer: http://backpack.tf/profiles/76561198092471230#!/compare/1471651200/1471737600

X = 15

Sale 4:

Sold for Aces Brigade Helm

No outpost

Seller: http://backpack.tf/profiles/76561198093577036#!/compare/1469577600/1471651200

Buyer: http://backpack.tf/profiles/76561198114671331#!/compare/1471132800/1471651200

Brigade Helm looks quite outdated, would probably be better off being used for the Brigade instead

No current sellers

Sales at:

15

16

17

Going with 15-17