Using last month.

Sale 1:

Sold for 7 keys

Seller: https://backpack.tf/profiles/76561198335791307#!/compare/1537228800/1537315200

Buyer: https://backpack.tf/profiles/76561198155219465#!/compare/1537228800/1537315200

https://backpack.tf/item/6260602053

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

X = 7

Sales 2-5:

Sold for $11.48, $11.28, $11.29, & $7 on marketplace.tf.

https://marketplace.tf/items/1108;5;u3001

https://backpack.tf/item/6271244696

http://manic.tf/keyprice/?at=2018-08-22 - $11.48 / $1.9 = 6.04 keys

http://manic.tf/keyprice/?at=2018-08-28 - $11.28 / $1.88 = 6.00 keys

http://manic.tf/keyprice/?at=2018-09-02 - $11.29 / $1.88 = 6.01 keys

http://manic.tf/keyprice/?at=2018-09-18 - $7 / $1.85 = 3.78 keys

X = 6 (3x)

X = 4

Sale 6:

Sold for 5 keys + 16 ref

Seller: https://backpack.tf/profiles/76561198312583107#!/compare/1534723200/1534809600

Buyer: https://backpack.tf/profiles/76561198199217834#!/compare/1534809600/1534896000

https://backpack.tf/item/2945014598

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

X = 5.5

Other Sales:

https://backpack.tf/item/5764040518 (#1)

https://backpack.tf/item/3075598080 (#2)

https://backpack.tf/item/2815440587 (#3)

https://backpack.tf/item/2775386709 (#4 & #5)

https://backpack.tf/item/2730773319 (#6 & #7)

https://backpack.tf/item/2705804159 - tf2mart stuff

#1

SCM

https://backpack.tf/profiles/76561198290033158#!/compare/1534723200/1534896000

https://backpack.tf/profiles/76561198156985344#!/compare/1534982400/1535068800

#2

SCM

https://backpack.tf/profiles/76561198037368910#!/compare/1535068800/1535155200

https://backpack.tf/profiles/76561198077408118#!/compare/1535068800/1535414400

#3

Untraceable

https://backpack.tf/profiles/76561198334598690#!/compare/1535155200/1535241600

https://backpack.tf/profiles/76561198845695370#!/compare/1535241600/1535328000 (no better compare link available)

https://i.imgur.com/vNFIyJM.png - Labelled as 5 keys + 16 ref

#4

Untraceable

https://backpack.tf/profiles/76561198154685875#!/compare/1535155200/1536969600

https://backpack.tf/profiles/76561198309220407#!/compare/1535414400/1536105600

#5

Bulk for Burning Base Metal Billycock

https://backpack.tf/profiles/76561198309220407#!/compare/1536710400/1536796800

https://backpack.tf/profiles/76561198074924284#!/compare/1536624000/1536710400

#6

Untraceable

https://backpack.tf/profiles/76561198239693272#!/compare/1535068800/1535155200

https://backpack.tf/profiles/76561198411234237#!/compare/1534982400/1535155200

#7

Bulk

https://backpack.tf/profiles/76561198411234237#!/compare/1535155200/1535241600

https://backpack.tf/profiles/76561198131658546#!/compare/1535068800/1536019200

Sellers:

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

https://backpack.tf/item/2775386709 - 12 keys

Sales at:

4

5.5

6 (3x)

7

4 is low, excluding 4.

Going with 5.5-7.