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

Sales 1 & 2:

Sold for $12.17 and $12.20 on marketplace.tf

https://marketplace.tf/items/1116;5;u3006

http://manic.tf/keyprice/?at=2018-09-01 - $12.17 / $1.88 = 6.47 keys

http://manic.tf/keyprice/?at=2018-09-04 - $12.2 / $1.88 = 6.49 keys

Couldn’t find item histories.

X = 6.5 (2x)

Sale 3:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198311498888#!/compare/1535760000/1535846400

Buyer: https://backpack.tf/profiles/76561198447542798#!/compare/1534982400/1535846400

https://backpack.tf/item/6077265870

X = 5

Sale 4:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198180662462#!/compare/1537315200/1537401600

Buyer: https://backpack.tf/profiles/76561198349787733#!/compare/1537401600/1537920000

https://backpack.tf/item/2802829750

X = 5

Sale 5:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1538265600/1538352000

Buyer: https://backpack.tf/profiles/76561198264591866#!/compare/1538352000/1538524800

https://backpack.tf/item/4441340035

X = 5

Sale 6:

Sold to a bot for 4 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198122554260#!/compare/1535760000/1535846400

Buyer: https://backpack.tf/profiles/76561198833317554#!/compare/1535673600/1535760000

https://backpack.tf/item/5154092112

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

X = 4.5

Sale 7:

Sold to a bot for 4 keys + 12 ref?

Seller: https://backpack.tf/profiles/76561198345126766#!/compare/1536451200/1538006400

Buyer: https://backpack.tf/profiles/76561198334598690#!/compare/1536451200/1536537600

https://backpack.tf/item/4441340035

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

Only the 12 ref matches, but since we have the exact amount of metal, I doubt bot.tf is wrong here.

X = 4.5

Sale 8:

Sold to a bot for 4 keys + 12 ref

Seller: https://backpack.tf/profiles/76561198264591866#!/compare/1538611200/1538697600

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

https://backpack.tf/item/4441340035

https://i.imgur.com/8G3X9UI.png

X = 4.5

Sale 9:

Sold to a bot for 4 keys + 8.11 ref?

Seller: https://backpack.tf/profiles/76561198040810040#!/compare/1536624000/1537315200

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

https://backpack.tf/item/2689067925

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

Only 2 keys and 8.11 ref match, but since we have the exact amount of metal, I doubt bot.tf is wrong here.

X = 4

Other Sales:

https://backpack.tf/item/4232904314 (#1, #4 & #5, rest appear to have been gambled)

https://backpack.tf/item/2690510074 (#2 & #3)

https://backpack.tf/item/3366746786 (#6)

https://backpack.tf/item/3985930861 - mannco stuff

https://backpack.tf/item/3170083892 - marketplace stuff/transferred between bot and bot owner

https://backpack.tf/item/2928237419 - not unusual at the time

https://backpack.tf/item/2701689244 - tf2mart

#1

Bulk

https://backpack.tf/profiles/76561198044010685#!/compare/1536364800/1536451200

https://backpack.tf/profiles/76561198258264645#!/compare/1536364800/1536451200

#2

Untraceable

https://backpack.tf/profiles/76561198149688091#!/compare/1535932800/1536019200

https://backpack.tf/profiles/76561198833317554#!/compare/1535932800/1536019200

https://i.imgur.com/HMjbhSj.png - labelled as 4 keys + 18 ref. 21 ref matches here.

#3

Bulk for vivid boonie?

https://backpack.tf/profiles/76561198830746853#!/compare/1535932800/1536019200

https://backpack.tf/profiles/76561198149688091#!/compare/1535932800/1536019200

https://i.imgur.com/0tL7BHg.png - labelled as 5 keys + 18 ref

#4

Untraceable

https://backpack.tf/profiles/76561198258264645#!/compare/1538179200/1538265600

https://backpack.tf/profiles/76561198833008516#!/compare/1537315200/1538265600

#5

Untraceable, labelled as 4 keys + 12 ref on bot.tf

https://backpack.tf/profiles/76561198094730000#!/compare/1538179200/1538265600

https://backpack.tf/profiles/76561198312583107#!/compare/1538265600/1539216000

https://i.imgur.com/8G3X9UI.png

#6

Untraceable

https://backpack.tf/profiles/76561198023184551#!/compare/1538956800/1539129600

https://backpack.tf/profiles/76561198164651434#!/compare/1538611200/1539043200

Most likely sold for 4 keys + 19.66 ref, which would fit in the range anyways.

Sellers:

https://i.imgur.com/0V7M8CO.png

https://i.imgur.com/1TiPHxd.png

https://backpack.tf/item/3366746786 - ~4.7 keys

https://backpack.tf/item/2689067925 - ~4.72 keys

https://backpack.tf/item/4232904314 - ~4.77 keys

https://backpack.tf/item/4439049493 - ~4.9 keys

https://backpack.tf/item/4441340035 - 5 keys

https://backpack.tf/item/2802829750 - 5 keys

Sales at:

4

4.5 (3x)

5 (3x)

6.5 (2x)

6.5 is an outlier given the cheaper sellers. Excluding 6.5.

4 is too low given the quickbuyer at ~4.3. https://i.imgur.com/iSZTa8t.png

Going with 4.5-5.