Using last month.

Sale 1:

Sold for $18.99 on marketplace.tf

https://marketplace.tf/items/30845;5;u3001

Can’t find the item’s history.

http://manic.tf/keyprice/?at=2018-09-09 - $18.99 / $1.87 = 10.16 keys

X = 10

Sale 2:

Sold for 8 keys + 17 ref

Seller: https://backpack.tf/profiles/76561198833317554#!/compare/1535155200/1535241600

Buyer: https://backpack.tf/profiles/76561198076523282#!/compare/1535155200/1535241600

https://backpack.tf/item/7050567524

https://i.imgur.com/2AIKm6x.png

X = 8.5

Sale 3:

Sold to a bot for 7 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198101731925#!/compare/1535241600/1535328000

Buyer: https://backpack.tf/profiles/76561198833317554#!/compare/1535241600/1535328000

https://backpack.tf/item/6252912502

https://i.imgur.com/2AIKm6x.png

X = 7.5

Other Sales:

https://backpack.tf/item/6436892724 - Not unusual at the time

https://backpack.tf/item/6343847589 (#1)

#1

Untraceable

https://backpack.tf/profiles/76561198335791307#!/compare/1535673600/1535760000

https://backpack.tf/profiles/76561198335791307#!/compare/1535673600/1535760000

https://i.imgur.com/2AIKm6x.png - Labelled as 8 keys

Sellers:

https://i.imgur.com/Qnywc0d.png

https://backpack.tf/item/6343847589 - 10 keys

Sales at:

7.5

8.5

10

Current highest buyer is 6. Should be fine to keep 7.5.

Going with 7.5-10.