Sale 1:

Sold for 17 keys + ~1 key in items

http://www.tf2outpost.com/trade/28819528 - B/O of 18

Seller: http://backpack.tf/profiles/76561198021055079#!/compare/1489190400/1489276800

Buyer: http://backpack.tf/profiles/76561198019019392#!/compare/1489276800/1489363200

http://imgur.com/a/plSuw

X = 18

Sale 2:

Sold in ~2-3 days for 16 keys

http://www.tf2outpost.com/trade/28879143 - B/O of 16

Seller: http://backpack.tf/profiles/76561198237292756#!/compare/1492387200/1492560000

Buyer: http://backpack.tf/profiles/76561198145498436#!/compare/1492473600/1492560000

X = 16

Sale 3:

Quicksold for 15 keys

No outpost

Seller: http://backpack.tf/profiles/76561198344933095#!/compare/1492128000/1492214400

Buyer: http://backpack.tf/profiles/76561198237292756#!/compare/1492214400/1492300800

X = 15

Sale 4:

Quicksold for 13 keys + 1 ref

Seller: http://backpack.tf/profiles/76561198082642255#!/compare/1488499200/1488585600

Buyer: http://backpack.tf/profiles/76561198305168523#!/compare/1488499200/1488585600

X = 13

Sellers:

18 keys:

http://i.imgur.com/D7Ddt1a.png - 19 days

http://backpack.tf/u/76561198080179568

http://www.tf2outpost.com/trade/28907213

http://i.imgur.com/qDQHEzi.png

http://backpack.tf/u/76561198059691615

Sales at:

13

15

16

18

All sales at 16 and less sold pretty quickly. Excluding 16 and under.

Going with 18 flat.