Using last month.

Sale 1:

Sold for 25 keys

Seller: https://backpack.tf/profiles/76561198045174236#!/compare/1537142400/1537228800

Buyer: https://backpack.tf/profiles/76561198037731984#!/compare/1535846400/1537142400

https://backpack.tf/item/3270431362

X = 25

Sale 2:

Sold for $39.98 on marketplace.tf

https://marketplace.tf/items/30570;5;u3006

http://manic.tf/keyprice/?at=2018-10-13 - $39.98 / $1.77 = 22.59 keys

X = 23

Sale 3:

Sold to a bot for 14 keys + 2 ref?

Seller: https://backpack.tf/profiles/76561198098325034#!/compare/1537833600/1537920000

Buyer: https://backpack.tf/profiles/76561198295122993#!/compare/1537833600/1537920000

https://backpack.tf/item/3554579317

https://i.imgur.com/ZV6vK2s.png

Only 11 keys and 2 ref match. Looks like the seller sold a key for ref, and the other 2 keys are untraceable. Should be fine to assume bot.tf is correct here.

X = 14

Other Sales:

https://backpack.tf/item/3554579317 (#1)

https://backpack.tf/item/6321417738 - marketplace

https://backpack.tf/item/3344260095 - marketplace

#1

SCM

https://backpack.tf/profiles/76561198382589541#!/compare/1538092800/1538179200

https://backpack.tf/profiles/76561198064788088#!/compare/1538179200/1538265600

Sellers:

https://i.imgur.com/n208L2i.png

https://backpack.tf/item/3341301268 - 20 keys

https://backpack.tf/item/3269687729 - 24 keys

https://backpack.tf/item/3568349790 - 25 keys

https://backpack.tf/item/3554579317 - 28 keys

Sales at:

14

23

25

14 went to a bot, and doesn’t appear to be a common trading point. Excluding 14.

The seller at 20 is rather young, and hasn’t been bumped in a day. Excluding for now.

Going with 23-25.