Sale 1:

Sold for 18 keys

http://www.tf2outpost.com/trade/28446990 - B/O of 18

Seller: http://backpack.tf/profiles/76561198018528305#!/compare/1473552000/1473638400

Buyer: http://backpack.tf/profiles/76561198058249606#!/compare/1472947200/1474934400

X = 18

Sale 2: (dupe)

Sold for 18 keys

http://www.tf2outpost.com/trade/28540085 - B/O of 18

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1476489600/1476576000

Buyer: http://backpack.tf/profiles/76561198114393947#!/compare/1476489600/1476576000

http://imgur.com/a/c6gI4

X = 18

Sale 3: (dupe)

Sold for 17 keys in ~2-3 days

No outpost

Seller: http://backpack.tf/profiles/76561198061376237#!/compare/1473120000/1473206400

Buyer: http://backpack.tf/profiles/76561198122038981#!/compare/1473206400/1473379200

X = 17

Sales at:

17

18 (2x)

No current sellers

Purchased for 15 keys via my buy order

Excluding sale at 17 as it is a dupe sale, it sold in a short-ish amount of time, and sales at 18 are more common.

Going with 18 flat