Sale 1:

Sold for 20 keys

Seller: https://backpack.tf/profiles/76561198136650804#!/compare/1522800000/1522886400

Buyer: https://backpack.tf/profiles/76561198167700518#!/compare/1522800000/1522886400

X = 20

Sale 2:

Sold with ~2 keys in items for 18 keys

Seller: https://backpack.tf/profiles/76561198254375254#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198052264884#!/compare/1523145600/1523232000

X = 16

Sale 3:

Sold for Steaming Universal Translator + 1 key

No outpost

Seller: https://backpack.tf/profiles/76561198043989712#!/compare/1522886400/1522972800

Buyer: https://backpack.tf/profiles/76561198254375254#!/compare/1522886400/1522972800

Should be used on the Translator.

Sale 4:

Sold with 2 keys for Strange Orbiting Fire Field Practice

Seller: https://backpack.tf/profiles/76561198167700518#!/compare/1522886400/1522972800

Buyer: https://backpack.tf/profiles/76561198043989712#!/compare/1522800000/1522886400

Should be used on the Practice.

Sellers:

~19 keys: – Scrap.tf

https://i.imgur.com/sKm21VL.png

https://backpack.tf/item/4173131801

21 keys:

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

https://backpack.tf/item/4081878769

26 keys:

https://i.imgur.com/BF9rj5B.png

https://backpack.tf/item/4227092170

Sales at:

16

20

Excluding 16, as it seems pretty low. The buyer of that sale sold it to scrap.tf for at least 17 keys:

https://backpack.tf/profiles/76561198052264884#!/compare/1523232000/1523577600

Scrap.tf been selling at 19 for nearly a month, including 19 in the range.

Going with 19-20.