Using last month.

Sale 1:

Sold for 8 keys

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1558742400/1558828800

Buyer: https://backpack.tf/profiles/76561198049322925#!/compare/1558742400/1558828800

https://backpack.tf/item/7412893927

https://i.imgur.com/tHpAitF.png - bot.tf confirms

X = 8

Sale 2:

Sold for 7 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198305494036#!/compare/1558742400/1558828800

Buyer: https://backpack.tf/profiles/76561198049322925#!/compare/1558742400/1558828800

https://backpack.tf/item/6812299514

https://i.imgur.com/tHpAitF.png - bot.tf confirms

X = 7.5

Sale 3:

Sold for 6 keys

Seller: https://backpack.tf/profiles/76561198278546360#!/compare/1559001600/1559088000

Buyer: https://backpack.tf/profiles/76561198197118257#!/compare/1558915200/1559088000

https://backpack.tf/item/7412893927

Everything between this and sale 1 in the item history was gambled.

X = 6

Marketplace Sales:

https://marketplace.tf/items/30921;5;u3005

$17.99 & $19.79. The sales are gonna end up being 10+ keys, way too high compared to sellers.

Other Sales:

https://backpack.tf/item/7606314498 (#1)

https://backpack.tf/item/6812299514 (#2)

https://backpack.tf/item/7420775027 - scrap

https://backpack.tf/item/7211341702 - not unusual at the time

https://backpack.tf/item/6689933619 - gambled

https://backpack.tf/item/6477468865 - marketplace

#1

Bulk for Purple Energy Cone

https://backpack.tf/profiles/76561198119429319#!/compare/1559001600/1559088000

https://backpack.tf/profiles/76561198120672630#!/compare/1558915200/1559088000

#2

Untraceable

https://backpack.tf/profiles/76561198037674114#!/compare/1558828800/1559088000

https://backpack.tf/profiles/76561198334598690#!/compare/1559088000/1559174400

#3

Sold for 4 keys?

https://backpack.tf/profiles/76561198963003558#!/compare/1558915200/1559001600

https://backpack.tf/profiles/76561198802465612#!/compare/1558915200/1559001600

https://i.imgur.com/tHpAitF.png - Was buying for 4 keys + 20.11 ref, according to bot.tf.

Fairly certain the seller is a scammer alt, so it should be fine to disregard this sale.

Sellers:

https://i.imgur.com/LdVDw6L.png

https://i.imgur.com/7O0WbTp.png

https://backpack.tf/item/6465155435 - ~7.75 keys

https://backpack.tf/item/6638416961 - ~7.8 keys

https://backpack.tf/item/6462117485 - 11 keys

https://backpack.tf/item/7535466056 - 11 keys

https://backpack.tf/item/7606314498 - 14 keys

https://backpack.tf/item/7560269847 - 14 keys

Sales at:

6

7.5

8

Last suggestion, 6 was excluded due to the buyer at 6, but this buyer is now gone, so it should be fine to keep 6.

2 sellers below 8, excluding 8.

Going with 6-7.5