Sale 1:

Sold for Max’s Head (23 at the time)

No outpost

Seller: http://backpack.tf/profiles/76561197999873164#!/compare/1470355200/1470441600

Buyer: http://backpack.tf/profiles/76561198122445210#!/compare/1470355200/1470441600

X = 23

Sale 2:

Sold with Ballonibouncer (2) for Sulphurous Bombing Run (21)

http://www.tf2outpost.com/trade/28456708 - No B/O

Seller: http://backpack.tf/profiles/76561198173747038#!/compare/1473033600/1473206400

Buyer: http://backpack.tf/profiles/76561198125704679#!/compare/1473120000/1473206400

X = 19

Sale 3:

Sold for 18 keys

http://www.tf2outpost.com/trade/28414961 - B/O of 18, seller confirms sale (legend)

Seller: http://backpack.tf/profiles/76561198056793444#!/compare/1472083200/1472169600

Buyer: http://backpack.tf/profiles/76561198173747038#!/compare/1472083200/1472169600

X = 18

Sale 4: (dupe)

Sold for 18 keys

http://www.tf2outpost.com/trade/28477084 - B/O of 18

Seller: http://backpack.tf/profiles/76561198035884807#!/compare/1475884800/1475971200

Buyer: http://backpack.tf/profiles/76561198103899658#!/compare/1475798400/1476403200

X = 18

Sale 5:

Sold for ~8 keys in items

No outpost

Seller: http://backpack.tf/profiles/76561198253548530#!/compare/1473033600/1473552000

Buyer: http://backpack.tf/profiles/76561198069090237#!/compare/1473465600/1473552000

X = 8

Sellers:

17 keys:

http://www.tf2outpost.com/trade/28565363 - young

http://i.imgur.com/C5Gh21r.jpg

http://backpack.tf/u/76561198049884052

20 keys:

http://www.tf2outpost.com/trade/28517646

http://backpack.tf/u/76561198095335435

http://i.imgur.com/upBquHB.png - 24 days, listed at 22 here

http://www.tf2outpost.com/trade/28516721

http://backpack.tf/u/76561198176125115

21 keys:

http://i.imgur.com/ZmGv965.png - 3 days

http://backpack.tf/u/76561198086312574

Offers:

http://www.tf2outpost.com/trade/28463345

http://backpack.tf/u/76561198060927833

Sales at:

8

18 (2x)

19

23

8 is obviously an outlier

Sale at 23 looks like a bit of an outlier, with a large amount of sales in the 18-19 range, and the amount of sellers for less than 23

Going with 18-19.