Using last month.

Sale 1:

Sold for $18.85 on marketplace.tf

https://marketplace.tf/items/30839;5;u3001

Couldn’t find the item’s history

http://manic.tf/keyprice/?at=2018-09-11 - $18.85 / $1.87 = 10.08 keys

X = 10

Sale 2:

Sold to a bot for 6 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198119609795#!/compare/1535587200/1535846400

Buyer: https://backpack.tf/profiles/76561198334598690#!/compare/1535760000/1535846400

https://backpack.tf/item/6061701587

https://i.imgur.com/m88nAtl.png

X = 6.5

Other Sales:

https://backpack.tf/item/6738585896 (#1 & #2)

https://backpack.tf/item/6054151243 - not unusual at the time

https://backpack.tf/item/6934388738 - not unusual at the time

https://backpack.tf/item/6839924136 - tf2mart stuff

https://backpack.tf/item/5866025809 - scammers

#1

Untraceable

https://backpack.tf/profiles/76561198287222419#!/compare/1535760000/1536019200

https://backpack.tf/profiles/76561198332277454#!/compare/1535673600/1536019200

#2

Untraceable

https://backpack.tf/profiles/76561198332277454#!/compare/1535673600/1536019200

https://backpack.tf/profiles/76561198363395191#!/compare/1536019200/1536105600

Sellers:

https://i.imgur.com/lD4JIHk.png

https://backpack.tf/item/6738585896 - 9 keys

https://backpack.tf/item/6061701587 - 10 keys

Sales at:

6.5

10

The sale at 6.5 is the only one in the last month, and bot.tf shows the most recent purchase around this price was around 2 months prior to the one listed here. With 4 buyers at ~6.5, it should be safe to assume that 6.5 isn’t a common trading point, and exclude it here.

Using the seller at 9 for the low-end.

Going with 9-10.