Using last month.

Sale 1:

Sold for $12.28 on marketplace.tf

https://marketplace.tf/items/1112;5;u3003

https://manic.tf/keyprice/?at=2018-10-12 - $12.28 / $1.78 = 6.90 keys

https://backpack.tf/item/3881494642

X = 7

Sale :

Sold to a quickbuyer for 5 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198184866919#!/compare/1537488000/1537660800

Buyer: https://backpack.tf/profiles/76561198130990555#!/compare/1537574400/1537660800

https://backpack.tf/item/2734181648

X = 5.5

Other Sales:

https://backpack.tf/item/5388128518 (#1)

https://backpack.tf/item/4440167181 - tf2mart

https://backpack.tf/item/6427431747 - tf2mart

https://backpack.tf/item/6085960979 - tradeit and other stuff

https://backpack.tf/item/2693389479 - marketplace

#1

Untraceable

https://backpack.tf/profiles/76561198286650376#!/compare/1538352000/1539043200

https://backpack.tf/profiles/76561198333384862#!/compare/1536969600/1539388800

Sellers:

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

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

https://backpack.tf/item/6085960979 - 6 keys

https://backpack.tf/item/2761271227 - 8 keys

https://backpack.tf/item/2847478783 - ~8.38 keys

https://backpack.tf/item/2737195586 - ~8.38 keys

https://backpack.tf/item/2691675803 - 15 keys

https://backpack.tf/item/4318068485 - ~17.5 keys

Sales at:

5.5

7

5.5 went to a quickbuyer and doesn’t appear to be a common trading point, but given the only other sale we have here is a whole key higher than the seller at 6, it would probably be better to keep 5.5 in the range.

Going with 5.5-7.