Using sales that were less than a month old when I started making this suggestion (27/9)

Sale 1:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198021055079#!/compare/1536019200/1536105600

Buyer: https://backpack.tf/profiles/76561198202534973#!/compare/1535932800/1536192000

https://backpack.tf/item/5768702275

X = 7

Sale 2:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1536537600/1536624000

Buyer: https://backpack.tf/profiles/76561198158294758#!/compare/1536364800/1536624000

https://backpack.tf/item/5090736837

X = 7

Sale 3:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1538006400/1538092800

Buyer: https://backpack.tf/profiles/76561198118080718#!/compare/1538006400/1538179200

https://backpack.tf/item/7014710100

X = 7

Sales 4-7:

Sold for $11.99, $10.58, $10.59, & $6.20 on marketplace.tf

https://marketplace.tf/items/30762;5;u3006

https://backpack.tf/item/6576883302 - No B/O

http://manic.tf/keyprice/?at=2018-09-13 - $11.99 / $1.87 = 6.41 keys

http://manic.tf/keyprice/?at=2018-09-29 - $10.58 / $1.84 = 5.75 keys

http://manic.tf/keyprice/?at=2018-10-07 - $10.59 / $1.8 = 5.88 keys

http://manic.tf/keyprice/?at=2018-09-17 - $6.2 / $1.85 = 3.35 keys

X = 6.5

X = 6 (2x)

X = 3.5

Sale 8:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198305676157#!/compare/1538006400/1538092800

Buyer: https://backpack.tf/profiles/76561198088822281#!/compare/1538006400/1538092800

https://backpack.tf/item/5090736837

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

X = 5

Sale 9:

Sold for 4 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198060113354#!/compare/1537056000/1537142400

Buyer: https://backpack.tf/profiles/76561198137923664#!/compare/1537056000/1537142400

https://backpack.tf/item/7145523674

X = 4.5

Sale 10:

Sold for 4 keys + 20 ref?

Seller: https://backpack.tf/profiles/76561198158294758#!/compare/1537142400/1537315200

Buyer: https://backpack.tf/profiles/76561198305676157#!/compare/1537142400/1537228800

https://backpack.tf/item/5090736837

https://i.imgur.com/24D3ocd.png

Only 1 key and 19 ref can be seen here. Since we have close to the right amount of ref, and since it’s clear that more pure was involved, but later spent on other items that day, it should be fine to use this sale.

X = 4.5

Other Sales:

https://backpack.tf/item/7145523674 (#1)

https://backpack.tf/item/7014710100 (#2 & #3)

https://backpack.tf/item/6825796934 (#4)

https://backpack.tf/item/6576883302 (#5)

https://backpack.tf/item/6334993923 (#6 & #7)

https://backpack.tf/item/5047675689 (#8)

https://backpack.tf/item/5032975690 (#9 & #12)

https://backpack.tf/item/4964127964 (#10)

https://backpack.tf/item/6415298861 (#11)

https://backpack.tf/item/4986241455 - (#13)

https://backpack.tf/item/6344672563 - not unusual at the time)

https://backpack.tf/item/5008159584 - not unusual at the time

https://backpack.tf/item/5323079432 - tradeit stuff

#1

Bulk for Kill-a-Watt Front Runner

https://backpack.tf/profiles/76561198443129839#!/compare/1536969600/1537056000

https://backpack.tf/profiles/76561198060113354#!/compare/1536969600/1537056000

#2

SCM

https://backpack.tf/profiles/76561198185232237#!/compare/1534204800/1536624000

https://backpack.tf/profiles/76561198095562107#!/compare/1535241600/1536796800

#3

Untraceable

https://backpack.tf/profiles/76561198095562107#!/compare/1537488000/1538006400

https://backpack.tf/profiles/76561198334598690#!/compare/1537401600/1537488000

https://i.imgur.com/24D3ocd.png - labelled as 4 keys + 14 ref

#4

Untraceable

https://backpack.tf/profiles/76561198351166269#!/compare/1537315200/1537574400

https://backpack.tf/profiles/76561198363395191#!/compare/1537574400/1537660800

This obviously sold for keys that were spent on the L’Etranger, but I can’t tell how many it went for.

#5

Untraceable

https://backpack.tf/profiles/76561198095759933#!/compare/1536883200/1537920000

https://backpack.tf/profiles/76561198312583107#!/compare/1536796800/1536883200

https://i.imgur.com/24D3ocd.png - labelled as 4 keys + 14 ref

#6

SCM

https://backpack.tf/profiles/76561198419251164#!/compare/1535500800/1535760000

https://backpack.tf/profiles/76561198256703280#!/compare/1535241600/1536019200

#7

Untraceable

https://backpack.tf/profiles/76561198256703280#!/compare/1536019200/1537833600

https://backpack.tf/profiles/76561198363395191#!/compare/1537833600/1537920000

#8

Bulk for Aus Knife

https://backpack.tf/profiles/76561198127045035#!/compare/1536796800/1536883200

https://backpack.tf/profiles/76561198276194006#!/compare/1536796800/1536883200

#9

SCM

https://backpack.tf/profiles/76561198242137378#!/compare/1535846400/1538006400

https://backpack.tf/profiles/76561198336407054#!/compare/1536710400/1536883200

#10

Untraceable

https://backpack.tf/profiles/76561198082838827#!/compare/1536883200/1536969600

https://backpack.tf/profiles/76561198080179568#!/compare/1536883200/1536969600

#11

dUMPED ON SCRAP?!?!?! for 5 keys + some metal

https://backpack.tf/profiles/76561198062734990#!/compare/1539129600/1539216000

https://backpack.tf/profiles/76561198135388328#!/compare/1539129600/1539216000

#12

Untraceable, labelled as 4 keys and 14 ref on bot.tf

https://backpack.tf/profiles/76561198336407054#!/compare/1538611200/1538697600

https://backpack.tf/profiles/76561198334598690#!/compare/1538611200/1538697600

#13

Untraceable, labelled as 4 keys and 20 ref on bot.tf

https://backpack.tf/profiles/76561198110254941#!/compare/1538006400/1538092800

https://backpack.tf/profiles/76561198305676157#!/compare/1538006400/1538092800

Sellers:

https://i.imgur.com/fl9cjgp.png

https://backpack.tf/item/6334993923 - ~4.72 keys

https://backpack.tf/item/4986241455 - ~4.72 keys

https://backpack.tf/item/6576883302 - ~4.77 keys

https://backpack.tf/item/5032975690 - 5 keys

Sales at:

3.5

4.5 (2x)

5

6 (2x)

6.5

7 (3x)

The sales above 5 are going to be countered by the 4 sellers at, and less than, 5 keys. Most of the sales at those prices either occurred before the market shifted to ~5 keys, or were marketplace.tf sales.

3.5 is an outlier, excluding.

4.5 is a common trading point, and despite buyers at around that price, is supported by current sellers.

Going with 4.5-5.