Using sales from April onwards.

Sale 1:

Sold for Australium Black Box (12)

No outpost

Seller: https://backpack.tf/profiles/76561198802566020#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198322421629#!/compare/1522108800/1523232000

X = 12

Sale 2:

Sol for $19.99 on Marketplace.tf

https://marketplace.tf/items/1120;5;u3007

19.99/1.98 = ~10.1

X = 10

Sale 3:

Sold for 7 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198061489334#!/compare/1522972800/1523059200

Buyer: https://backpack.tf/profiles/76561198528849893#!/compare/1522972800/1523059200

X = 7.5

Sale 4:

Sold for 6 keys + 4 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1522713600/1522800000

Buyer: https://backpack.tf/profiles/76561198263342776#!/compare/1522627200/1522800000

Bot.tf confirms: https://i.imgur.com/1XL0TYc.png

X = 6

Sale 5:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198120847838#!/compare/1523145600/1523491200

Untraceable, but bot.tf confirms: https://i.imgur.com/1XL0TYc.png

X = 6

Sale 6:

Sold for 5 keys + 23 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1524873600/1524960000

Buyer: https://backpack.tf/profiles/76561198300301404#!/compare/1524787200/1524960000

Untraceable, but bot.tf confirms: https://i.imgur.com/1XL0TYc.png

X = 6

Sellers:

8 keys:

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

https://backpack.tf/item/3346045721

10 keys:

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

https://backpack.tf/item/6639019229

14 keys:

https://i.imgur.com/JN9BQso.png

https://backpack.tf/item/3480773966

Sales at:

6 (3x)

7.5

10

12

Seller at 8 is a month old, excluding anything higher.

Going with 6-7.5.