Using last month

Sale 1:

Sold for 5 keys + 15 ref

Seller: https://backpack.tf/profiles/76561198285019378#!/compare/1523059200/1523232000

Buyer: https://backpack.tf/profiles/76561198267589362#!/compare/1523145600/1523232000

X = 5.5

Sales 2 & 3:

Bot.tf sales at 5 keys and 5 keys + 20 ref

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

Can’t find either of the sales

X = 5.5

X = 5

Sale 4:

Sold for $10 on Marketplace.tf

https://marketplace.tf/items/1116;5;u3008

https://backpack.tf/item/5999118659

10/1.99 = ~5.03

X = 5

Sale 5:

Sold for 5 keys

Seller: https://backpack.tf/profiles/76561198072277253#!/compare/1522454400/1522540800

Buyer: https://backpack.tf/profiles/76561198263801172#!/compare/1522454400/1522540800

X = 5

Sale 6:

Bulk for Ancient Codex Head Full of Hot Air

Seller: https://backpack.tf/profiles/76561198263801172#!/compare/1523145600/1523232000

Buyer: https://backpack.tf/profiles/76561198285019378#!/compare/1523059200/1523232000

Sellers:

~5.73 keys:

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

https://backpack.tf/item/5315409584

6 keys:

https://i.imgur.com/PveaCG0.png

https://backpack.tf/item/4272547066

7 keys:

https://i.imgur.com/fSk6RCY.png

https://backpack.tf/item/2771325301

9 keys:

https://i.imgur.com/C6CUHcm.png

https://backpack.tf/item/5243085608

10 keys:

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

https://backpack.tf/item/2702144281

15 keys:

https://i.imgur.com/3FnMHJh.png

https://backpack.tf/item/2724933389

Sales at:

5 (3x)

5.5 (2x)

Going with 5-5.5