Using last month.

Sales 1-3:

Sold for $10.13, $13, & $19.99 on marketplace.tf

https://marketplace.tf/items/1168;5;u3003

https://manic.tf/keyprice/?at=2018-09-17 - $10.13 / $1.85 = 5.48 keys

https://manic.tf/keyprice/?at=2018-09-20 - $13 / $1.84 = 7.07 keys

https://manic.tf/keyprice/?at=2018-10-11 - $19.99 / $1.78 = 11.23 keys

X = 11

X = 7

X = 5.5

Sale 4:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198024867201#!/compare/1539302400/1539388800

Buyer: https://backpack.tf/profiles/76561198355817251#!/compare/1539043200/1539388800

https://backpack.tf/item/4954187950

X = 10

Sale 5:

Sold for 9 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1537833600/1537920000

Buyer: https://backpack.tf/profiles/76561198105071091#!/compare/1537747200/1537920000

https://backpack.tf/item/6007702970

https://i.imgur.com/AlNV0WR.png

X = 9.5

Other Sales:

https://backpack.tf/item/6273286469 (#1)

https://backpack.tf/item/6061678344 (#2)

https://backpack.tf/item/6007702970 (#3 & #4)

https://backpack.tf/item/5189864875 - scammer

https://backpack.tf/item/5154153293 - marketplace

https://backpack.tf/item/5113268426 - not unusual at the time

#1

Untraceable

https://backpack.tf/profiles/76561198312583107#!/compare/1539216000/1539302400

https://backpack.tf/profiles/76561198056531196#!/compare/1539388800/1539475200

Probably sold to someone who gambled it away. Was gambled again afterwards.

#2

Untraceable

https://backpack.tf/profiles/76561198045883624#!/compare/1538697600/1538956800

https://backpack.tf/profiles/76561198061119329#!/compare/1538784000/1538870400

#3

Untraceable

https://backpack.tf/profiles/76561198105071091#!/compare/1538265600/1538352000

https://backpack.tf/profiles/76561198311195384#!/compare/1537920000/1538352000

#4

Untraceable

https://backpack.tf/profiles/76561198311195384#!/compare/1538784000/1538870400

https://backpack.tf/profiles/76561198334598690#!/compare/1538697600/1538784000

https://i.imgur.com/fmUtivK.png - labelled as 7 keys + 15 ref, was bought from the bot and dumped on scrap without being scanned afterwards too.

Sellers:

https://i.imgur.com/COHaM5o.png

https://backpack.tf/item/4959449360 - 10 keys

https://backpack.tf/item/6743912847 - 19 keys

Sales at:

5.5

7

9.5

10

11

11 is too high given the month unsold at 10. Excluding 11.

Buyers at ~7.5 keys, excluding anything below. https://i.imgur.com/ATGee0M.png

Going with 9.5-10.