Using last month.

Sale 1:

Sold for 4 keys + 12 ref

Seller: https://backpack.tf/profiles/76561198357046243#!/compare/1526774400/1526860800

Buyer: https://backpack.tf/profiles/76561198176510581#!/compare/1526688000/1526860800

Bot.tf confirms: https://i.imgur.com/PK2aHMj.png

X = 4.5

Sale 2:

Sold for 4 keys + 10 ref

Seller: https://backpack.tf/profiles/76561198334598690#!/compare/1526774400/1526860800

Buyer: https://backpack.tf/profiles/76561198116514071#!/compare/1526774400/1526860800

Bot.tf confirms: https://i.imgur.com/PK2aHMj.png

X = 4.5

Sale 3:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198076061300#!/compare/1527033600/1527120000

Buyer: https://backpack.tf/profiles/76561198798883073#!/compare/1527033600/1527292800

X = 4

Sale 4:

Sold to a bot for 4 keys + 6 ref

Seller: https://backpack.tf/profiles/76561198147977013#!/compare/1525132800/1527638400

Buyer: https://backpack.tf/profiles/76561198386441509#!/compare/1525046400/1525132800

Bot.tf confirms: https://i.imgur.com/PK2aHMj.png

X = 4

Sale 5:

Sold to a quickbuyer for 4 keys

Seller: https://backpack.tf/profiles/76561198275783474#!/compare/1526342400/1526428800

Buyer: https://backpack.tf/profiles/76561198048851745#!/compare/1526342400/1526428800

X = 4

Sale 6:

Sold with 7.66 ref for 4 keys

Seller: https://backpack.tf/profiles/76561198176510581#!/compare/1526860800/1526947200

Buyer: https://backpack.tf/profiles/76561198091114998#!/compare/1526860800/1526947200

X = 3.5

Sale 7:

Sold for 3 keys + 20.66 ref

Seller: https://backpack.tf/profiles/76561198347838878#!/compare/1526774400/1526860800

Buyer: https://backpack.tf/profiles/76561198147078303#!/compare/1526774400/1526860800

X = 3.5

Sale 8:

Sold to a bot for 3 keys + 0.33 ref

Seller: https://backpack.tf/profiles/76561198147078303#!/compare/1527120000/1527379200

Buyer: https://backpack.tf/profiles/76561198357046243#!/compare/1527033600/1527120000

X = 3

Sale 9:

Sold for $15 on Marketplace.tf

https://marketplace.tf/items/1114;5;u3004

Will end up being around 7.5 keys, which is significantly higher than the sellers at the moment. Excluding.

https://backpack.tf/item/2925361098 - seller was a scammer anyways.

Sale 10:

Bulk for Cloud 9 Bootleg Base Metal Billycock

Seller: https://backpack.tf/profiles/76561198798883073#!/compare/1527033600/1527292800

Buyer: https://backpack.tf/profiles/76561198146360194#!/compare/1527120000/1527292800

Sellers:

4 keys:

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

https://backpack.tf/item/5452771290

~4.4 keys:

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

https://backpack.tf/item/6783389822

5 keys:

https://i.imgur.com/Eh40AYU.png

https://backpack.tf/item/2818593838

https://backpack.tf/item/2793507318

6 keys:

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

https://backpack.tf/item/5629084265

7 keys:

https://i.imgur.com/HDo9MZK.png

https://backpack.tf/item/4110637328

9 keys:

https://i.imgur.com/0b3LZBE.png

https://backpack.tf/item/3730497980

10 keys:

https://i.imgur.com/foy1aOB.png

https://backpack.tf/item/2716975224

Sales at:

3

3.5 (2x)

4 (3x)

4.5 (2x)

3 looks low, excluding.

Going with 3.5-4.5.

sold for ~4.5 keys

https://backpack.tf/profiles/76561198357046243#!/compare/1527984000/1528156800

https://backpack.tf/profiles/76561198089330608#!/compare/1528070400/1528156800

https://i.imgur.com/uD104SK.png

sold for 4 keys

https://backpack.tf/profiles/76561198312265776#!/compare/1527811200/1527897600

https://backpack.tf/profiles/76561198400938562#!/compare/1527897600/1527984000

https://backpack.tf/item/6480010606

sold for ~4 keys after rounding

https://backpack.tf/profiles/76561198400938562#!/compare/1527897600/1527984000

https://backpack.tf/profiles/76561197999590170#!/compare/1527984000/1528070400

sold for 5 keys

https://backpack.tf/profiles/76561198386441509#!/compare/1528502400/1528588800

https://backpack.tf/profiles/76561198079466852#!/compare/1528502400/1528588800

don't know if i should be including 5, given all the cheaper sales.