Using last month.

Sale 1:

Sold for $15 on marketplace.tf

https://marketplace.tf/items/30816;5;u3007

http://manic.tf/keyprice/?at=2018-09-08 - $15 / $1.88 = 7.98 keys

X = 8

Sale 2:

Sold for 4 keys + Schadenfreude (~1)

Seller: https://backpack.tf/profiles/76561198227232564#!/compare/1538179200/1538265600

Buyer: https://backpack.tf/profiles/76561198138189083#!/compare/1538092800/1538265600

https://backpack.tf/item/5852578543

X = 5

Other Sales:

https://backpack.tf/item/6558650941 (#1)

https://backpack.tf/item/5852578543 (#2 & #3)

https://backpack.tf/item/5185471518 (#4)

https://backpack.tf/item/5213772440 - mannco

#1

Sold with 3 keys for Orbiting Fire Dead Cone

https://backpack.tf/profiles/76561198069813252#!/compare/1537056000/1537142400

https://backpack.tf/profiles/76561197975587947#!/compare/1536969600/1537056000

Shouldn’t be used here, might be able to use on the Cone.

#2

Bulk for Grail Broom

https://backpack.tf/profiles/76561198135398757#!/compare/1538006400/1538092800

https://backpack.tf/profiles/76561198227232564#!/compare/1538006400/1538092800

#3

Bulk for Circling TF Logo Sky High Fly Guy

https://backpack.tf/profiles/76561198138189083#!/compare/1538092800/1538265600

https://backpack.tf/profiles/76561198322002351#!/compare/1538006400/1538438400

#4

SCM

https://backpack.tf/profiles/76561198184539327#!/compare/1536192000/1536624000

https://backpack.tf/profiles/76561198091453852#!/compare/1536969600/1537574400

Sellers:

https://i.imgur.com/UolF4mO.png

https://backpack.tf/item/6528881285 - 6 keys

https://backpack.tf/item/5376745490 - ~6.18 keys

https://backpack.tf/item/6285463260 - ~6.23 keys

https://backpack.tf/item/6741525604 - ~6.23 keys

https://backpack.tf/item/5523926975 - 16 keys

Sales at:

5

8

8 is too high given the unsolds for cheaper. Excluding 8.

Going with 5 flat.