Using last month.

Sales 1 & 2:

Sold for $23 and $22.99 on marketplace.tf

https://marketplace.tf/items/167;5;u3004

22.99/1.95 = ~11.8

23/1.97 = ~11.7

X = 12 (2x)

Sale 3:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1532044800/1532131200

Buyer: https://backpack.tf/profiles/76561198390387069#!/compare/1532044800/1532217600

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/1chrqm4.png

Key histories below.

https://backpack.tf/item/7016535836

https://backpack.tf/item/7016535844

https://backpack.tf/item/7016535847

https://backpack.tf/item/7016535852

https://backpack.tf/item/7016535859

https://backpack.tf/item/7016535867

https://backpack.tf/item/7016535870

https://backpack.tf/item/7016535877

https://backpack.tf/item/7016535889

https://backpack.tf/item/7016535903

X = 10

Sale 4:

Sold for 10 keys?

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1532217600/1532304000

https://backpack.tf/item/6401411943 - buyer is untraceable currently.

Bot.tf confirms, but it is unreliable. https://i.imgur.com/1chrqm4.png

X = 10?

Sale 5:

Sold for 8 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198260549932#!/compare/1529884800/1529971200

Buyer: https://backpack.tf/profiles/76561198185339862#!/compare/1529539200/1530144000

X = 8

Sale 6:

Sold to a bot for 8 keys + 2 ref

Seller: https://backpack.tf/profiles/76561198185339862#!/compare/1530144000/1532476800

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1530403200/1530489600

Bot.tf confirms: https://i.imgur.com/1chrqm4.png

Only 1 key doesn’t appear on the seller’s end, but I can guarantee the missing key sold for metal, which can be seen in the compare link.

X = 8

Sale 7:

Sold to a bot for 8 keys + 2 ref?

Buyer: https://backpack.tf/profiles/76561198312583107#!/compare/1530482400/1530568800

https://backpack.tf/item/6401411943 - Seller is untraceable.

Bot.tf confirms, but it is unreliable. https://i.imgur.com/1chrqm4.png

X = 8?

Sale 8:

Sold to a bot for 8 keys + 1 scrap?

Seller: https://backpack.tf/profiles/76561198050494196#!/compare/1530748800/1531008000

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1530921600/1531008000

Untraceable. Bot.tf confirms, but it is unreliable. https://i.imgur.com/1chrqm4.png

X = 8?

Sale 9:

Sold for Orbiting Planets Towering Titanium Pillar of Hats

http://www.tf2outpost.com/trade/29694992 - B/O of 15

Seller: https://backpack.tf/profiles/76561198314620824#!/compare/1529884800/1529971200

Buyer: https://backpack.tf/profiles/76561198412265321#!/compare/1529798400/1529971200

This sale may be a bit high, and we have enough pure sales to work with here, so it would be best to just stick with them instead.

Sellers:

13 keys:

https://i.imgur.com/fB1qKOi.png

https://backpack.tf/item/5747957776

15 keys:

https://i.imgur.com/NGhNS1F.png

https://backpack.tf/item/3794358030

30 keys:

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

https://backpack.tf/item/6053467764

33 keys:

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

https://backpack.tf/item/1768038569

40 keys:

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

https://backpack.tf/item/4803979853

60 keys:

https://i.imgur.com/Zkp0ko0.png

https://backpack.tf/item/6336061065

Sales at:

8 (2x)

10

12 (2x)

And untraceable sales at 8 (2x) and 10.

Going with 8-12.