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

Sales 1-3:

Sold for $12.99, $6, and $6.02 on marketplace.tf

https://marketplace.tf/items/30614;5;u3006

http://manic.tf/keyprice/?at=2018-09-04 - $6.02 / $1.88 = 3.20 keys

http://manic.tf/keyprice/?at=2018-09-10 - $6 / $1.87 = 3.21 keys

http://manic.tf/keyprice/?at=2018-09-20 - $12.99 / $1.84 = 7.06 keys

https://backpack.tf/item/3806228650 - no b/o

Couldn’t find the others.

X = 7

X = 3 (2x)

Sale 4:

Sold for 4 keys + ~1 key in metal

Seller: https://backpack.tf/profiles/76561198175402807#!/compare/1537315200/1537660800

Buyer: https://backpack.tf/profiles/76561198264380993#!/compare/1537574400/1537660800

https://backpack.tf/item/3704626474

X = 5

Sale 5:

Sold for 4 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198363395191#!/compare/1537142400/1537228800

Buyer: https://backpack.tf/profiles/76561198181380673#!/compare/1537142400/1537228800

https://backpack.tf/item/6088620357

X = 4.5

Sale 6:

Sold for 4 keys + ~0.5 keys in metal

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1536796800/1536883200

Buyer: https://backpack.tf/profiles/76561198199310601#!/compare/1536796800/1537401600

https://backpack.tf/item/3842797928

X = 4.5

Sale 7:

Sold for 4 keys + 16 ref

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1538784000/1538870400

Buyer: https://backpack.tf/profiles/76561198120204836#!/compare/1538784000/1539129600

https://backpack.tf/item/3670150139

https://i.imgur.com/ZZT0HHU.png

X = 4.5

Sale 8:

Sold to a bot for 3 keys + 21 ref (keys on buyer’s end)

Seller: https://backpack.tf/profiles/76561198139171368#!/compare/1538092800/1538179200

Buyer: https://backpack.tf/profiles/76561198334598690#!/compare/1538092800/1538179200

https://backpack.tf/item/4401142838

https://i.imgur.com/ZZT0HHU.png

X = 3.5

Sale 9:

Sold for 122 ref (~3 keys)

Seller: https://backpack.tf/profiles/76561198343670225#!/compare/1538092800/1538179200

Buyer: https://backpack.tf/profiles/76561198267750483#!/compare/1538092800/1538179200

https://backpack.tf/item/5048905598

X = 3

Other Sales:

https://backpack.tf/item/7161665711 (#1)

https://backpack.tf/item/6655348267 (#2)

https://backpack.tf/item/6088620357 (#3)

https://backpack.tf/item/5393074354 (#4)

https://backpack.tf/item/3760141980 (#5)

https://backpack.tf/item/3704626474 (#6)

https://backpack.tf/item/3919878833 (#7)

https://backpack.tf/item/5048905598 (#8)

https://backpack.tf/item/4962672138 (#10)

https://backpack.tf/item/4937324544 - marketplace stuff

https://backpack.tf/item/4595779355 - marketplace stuff

https://backpack.tf/item/3668807218 - mannco stuff

#1

Untraceable

https://backpack.tf/profiles/76561198061277989 - compare wont update

https://backpack.tf/profiles/76561198284124072#!/compare/1537747200/1537833600

#2

SCM

https://backpack.tf/profiles/76561198056957730#!/compare/1535500800/1536710400

https://backpack.tf/profiles/76561198083165069#!/compare/1533168000/1536105600

#3

Untraceable

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

https://backpack.tf/profiles/76561198181380673#!/compare/1537920000/1538006400

#4

Untraceable

https://backpack.tf/profiles/76561198833990857#!/compare/1535587200/1536710400

https://backpack.tf/profiles/76561198373128844#!/compare/1535414400/1536796800

#5

Most likely traded between friends

https://backpack.tf/profiles/76561198414889398#!/compare/1535500800/1536796800

https://backpack.tf/profiles/76561198415564349#!/compare/1536278400/1536710400

#6

Untraceable

https://backpack.tf/profiles/76561198264380993#!/compare/1537660800/1537833600

https://backpack.tf/profiles/76561198335791307#!/compare/1537660800/1537747200

https://i.imgur.com/Kiv0BZj.png - labelled as 3 keys + 24 ref

#7

Untraceable

https://backpack.tf/profiles/76561198833317554#!/compare/1536012000/1536098400

https://backpack.tf/item/3919878833 - seller appears to be private, it isn’t the one listed here

https://i.imgur.com/Kiv0BZj.png - labelled as 4 keys + 2 ref

#8

Untraceable to the point where the history can’t be obtained

https://backpack.tf/profiles/76561198295122993#!/compare/1536789600/1536876000

https://i.imgur.com/Kiv0BZj.png - labelled as 4 keys + 2.33 ref

#9

Untraceable

https://backpack.tf/profiles/76561198267750483#!/compare/1538179200/1538697600

https://backpack.tf/profiles/76561198312583107#!/compare/1538092800/1538179200

#10

Untraceable

https://backpack.tf/profiles/76561198046450230#!/compare/1539043200/1539129600

https://backpack.tf/profiles/76561198017234762#!/compare/1539043200/1539129600

Sellers:

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

https://i.imgur.com/Y4XOCHA.png

https://backpack.tf/item/4710649584 - 4 keys

https://backpack.tf/item/4962672138 - ~4.39 keys

https://backpack.tf/item/5048905598 - ~4.41 keys

https://backpack.tf/item/6088620357 - ~4.41 keys

https://backpack.tf/item/3704626474 - ~4.52 keys

https://backpack.tf/item/4401142838 - 5 keys

https://backpack.tf/item/3608813629 - ~6.52 keys

https://backpack.tf/item/5299038838 - 12 keys

Sales at:

3 (3x)

3.5

4.5 (3x)

5

7

Due to the sellers, anything above 4.5 should be excluded. Excluding 5 and 7

Due to the buyers, anything below 3.5 should be excluded. Excluding 3.

3.5 may be the only traceable sale, but according to bot.tf, there are 2 more sales at ~3.5 (both untraceable), and there’s a seller at 4 in the classifieds, which goes against a flat price of 4.5. Keeping 3.5.

Going with 3.5-4.5.