Sale 1:

Sold for 23 keys

Seller: https://backpack.tf/profiles/76561197994697258#!/compare/1521590400/1521676800

Buyer: https://backpack.tf/profiles/76561198056132929#!/compare/1521504000/1521676800

X = 23

Sale 2:

Sold for Blizzardy Attendant (Suggestion at 14.5) + Strike Battin’ a Thousand (In-date at 5.5, seller less than 4.5, taking less than 5)

http://www.tf2outpost.com/trade/29629425 - B/O of 22

Seller: https://backpack.tf/profiles/76561198074178367#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198190835542#!/compare/1524268800/1524528000

https://i.imgur.com/IIuQmQU.png - Strike Seller

X = 19

Sale 3:

Sold with ref (~0.5) for Stormy Herald’s Helm (Suggestion at 20 below)

No outpost

Seller: https://backpack.tf/profiles/76561198190835542#!/compare/1524528000/1524614400

Buyer: https://backpack.tf/profiles/76561198170427507#!/compare/1524355200/1524614400

X = 19

Sale 4:

Sold for Miami Nights Big Chief (Suggestion at 16)

http://www.tf2outpost.com/trade/29641347 - B/O of 20

Seller: https://backpack.tf/profiles/76561198055622215#!/compare/1525737600/1525910400

Buyer: https://backpack.tf/profiles/76561198175643622#!/compare/1525737600/1525910400

X = 16

Sale 5:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198322876605#!/compare/1526169600/1526256000

Buyer: https://backpack.tf/profiles/76561198099272603#!/compare/1525996800/1526256000

X = 15

Sale 6:

Bulk for Purple Energy Sinner’s Shade

Seller: https://backpack.tf/profiles/76561198315895705#!/compare/1523664000/1524355200

Buyer: https://backpack.tf/profiles/76561198198079765#!/compare/1524268800/1524355200

Sale 7:

Sold for Spectral Balloonibouncer

Seller: https://backpack.tf/profiles/76561198170427507#!/compare/1524614400/1524960000

Buyer: https://backpack.tf/profiles/76561198055622215#!/compare/1524873600/1524960000

Can be used on the Balloonibouncer

Miami Chief:

https://backpack.tf/suggestion/5b02867044325a138b5142f7

Stormy Herald’s:

https://backpack.tf/suggestion/5b02f95a44325a3a8c0b2703

Blizzardy Attendant:

https://backpack.tf/suggestion/5b02f455cf6c75631e0c3c70

Sellers:

15 keys:

http://www.tf2outpost.com/trade/29656957

18 keys:

https://i.imgur.com/kYlO7Zk.png

https://backpack.tf/item/6519136623

20 keys:

https://i.imgur.com/P3vWKqW.png

https://backpack.tf/item/6552624939

24 keys:

https://i.imgur.com/IUkGLZO.png

https://backpack.tf/item/6465536877

26 keys:

https://i.imgur.com/QryjQLh.png

https://backpack.tf/item/6471177391

Sales at:

15

16

19 (2x)

23

23 looks like an outlier, and is pretty old now. Excluding.

Going with 15-19.