Sale 1:

Sold for 20 keys

No outpost

Seller: http://backpack.tf/profiles/76561198129088942#!/compare/1473638400/1473724800

Buyer: http://backpack.tf/profiles/76561198290343129#!/compare/1473724800/1473897600

X = 20

Sale 2:

Sold with ~4 keys in junk for Orbiting Planets Dr’s Dapper Topper

No outpost

Seller: http://backpack.tf/profiles/76561198188260109#!/compare/1467072000/1472342400

Buyer: http://backpack.tf/profiles/76561198021055079#!/compare/1472256000/1472342400

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

http://puu.sh/rg9yT/753d36455d.jpg

http://puu.sh/rg9ze/7af5adc921.jpg

X + 4 = 21.5

X = 18

Sale 3:

Sold for 14 keys

No outpost

Seller: http://backpack.tf/profiles/76561198187215751#!/compare/1470528000/1473465600

Buyer: http://backpack.tf/profiles/76561198129088942#!/compare/1473379200/1473465600

X = 14

Sale 4:

Sold for ~9 keys in junk

No outpost

Seller: http://backpack.tf/profiles/76561198290343129#!/compare/1473984000/1474243200

Buyer: http://backpack.tf/profiles/76561198148540751#!/compare/1473984000/1474070400

X = 9

Buyer at 18 keys:

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

http://backpack.tf/u/76561198215407981

Sellers:

18 keys:

http://www.tf2outpost.com/trade/28494115 - 1 day, unbumped

Sales at:

20

18

14

9

Buyer at 18 shows that anything at 18 and under is too low.

Going with 20 flat.