Sale 1:

Sold with 3 keys for Sunbeams Detective Noir (Mini at 55 below)

Seller: https://backpack.tf/profiles/76561198013517496#!/compare/1547942400/1548028800

Buyer: https://backpack.tf/profiles/76561198106849261#!/compare/1547596800/1548028800

https://backpack.tf/item/5972024292

X = 52

Sale 2:

Sold for Spec. KS Aus Medi Gun (~40 at the time) + 10 keys

Seller: https://backpack.tf/profiles/76561198150783829#!/compare/1542412800/1542499200

Buyer: https://backpack.tf/profiles/76561198124615050#!/compare/1542412800/1542499200

https://backpack.tf/item/5972024292

X = 50

Sale 3:

Sold for 25 keys + Stormy Storm Your Worst Nightmare (Suggestion at 15)

Seller: https://backpack.tf/profiles/76561198137642054#!/compare/1546473600/1546560000

Buyer: https://backpack.tf/profiles/76561198013517496#!/compare/1546473600/1546560000

https://backpack.tf/item/5972024292

X = 40

Sales 4-6:

Sold for $66 (2x) & $65.81 on marketplace

https://marketplace.tf/items/30177;5;u7

https://manic.tf/keyprice/?at=2018-11-11 - $66 / $1.77 = 37.29 keys (2x)

https://manic.tf/keyprice/?at=2018-12-10 - $65.81 / $1.67 = 39.41 keys

X = 39

X = 37 (2x)

Sale 7:

Quicksold to a quickbuyer for 38 quick keys

Seller: https://backpack.tf/profiles/76561198106849261#!/compare/1548028800/1548115200

Buyer: https://backpack.tf/profiles/76561198337734785#!/compare/1548028800/1548115200

X = 38

Sale 8:

Sold for Green Confetti Galvanized Gibus

Seller: https://backpack.tf/profiles/76561197982879578#!/compare/1545955200/1546041600

Buyer: https://backpack.tf/profiles/76561198137642054#!/compare/1545955200/1546128000

https://backpack.tf/item/5972024292

Using on the Gibus.

Sale 9:

Untraceable

Seller: https://backpack.tf/profiles/76561198066195760#!/compare/1541548800/1541635200

Buyer: https://backpack.tf/profiles/76561198399988181#!/compare/1541548800/1541635200

https://backpack.tf/item/6821935866

Sale 10:

Probably gambled or something

Seller: https://backpack.tf/profiles/76561198399988181#!/compare/1541548800/1541635200

Buyer: https://backpack.tf/profiles/76561198248296912#!/compare/1541548800/1541635200

https://backpack.tf/item/6821935866

Sale 11:

Bulk for Circling Heart Hong Kong Cone

Seller: https://backpack.tf/profiles/76561198078306332#!/compare/1544832000/1544918400

Buyer: https://backpack.tf/profiles/76561198858969285#!/compare/1543622400/1544918400

https://backpack.tf/item/7360594955

Stormy Storm Your Worst Nightmare:

https://backpack.tf/suggestion/5c59700944325a40ad2f05b0

Sunbeams Detective Noir Mini:

Sold for 55 keys

Seller: https://backpack.tf/profiles/76561198054758757#!/compare/1547424000/1547856000

Buyer: https://backpack.tf/profiles/76561198118605824#!/compare/1547769600/1547856000

https://backpack.tf/item/434215040

X = 55

Sellers:

https://i.imgur.com/5ptOry1.png

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

https://backpack.tf/item/5972024292 - 44 keys

https://backpack.tf/item/7071040367 - 48 keys

https://backpack.tf/item/7360594955 - 48 keys

https://backpack.tf/item/6821935866 - 53 keys

https://backpack.tf/item/5105824461 - 54 keys

https://backpack.tf/item/7449310274 - 58 keys

https://backpack.tf/item/4959394943 - 60 keys

https://backpack.tf/item/3475876584 - 60 keys

Sales at:

37 (2x)

38

39

40

50

52

The number of sales at ~40 outweigh the number of sales higher than 40, and there are 3 sellers that are fairly higher than these non-pure sales that counter them anyways. Also, the sale at 50 is ancient. Excluding 50 and 52.

Buyers go as high as ~39.2 keys, excluding 39 and below.

Going with 40 flat.