Using last month.

Sales 1 & 2:

Sold for $14.94 and $12.99 on marketplace.tf

https://marketplace.tf/items/1117;5;u3005

12.99/1.97 = ~6.59

14.94/1.95 = ~7.66

X = 7.5

X = 6.5

Sale 3:

Sold for 5 keys + 14 ref

Seller: https://backpack.tf/profiles/76561198048851745#!/compare/1530748800/1530835200

Buyer: https://backpack.tf/profiles/76561198446548676#!/compare/1530662400/1530748800

X = 5.5

Sale 4:

Sold for ~4 keys in metal/items

Seller: https://backpack.tf/profiles/76561198246590586#!/compare/1530662400/1530835200

Buyer: https://backpack.tf/profiles/76561198298979151#!/compare/1530748800/1530835200

X = 4

Sale 5:

Sold for 4 keys

Seller: https://backpack.tf/profiles/76561198328549805#!/compare/1532304000/1532476800

Buyer: https://backpack.tf/profiles/76561198122916546#!/compare/1532304000/1532476800

X = 4

Sellers:

6 keys:

https://i.imgur.com/nz6Npia.png

https://backpack.tf/item/5407209731

https://backpack.tf/item/6211974518

12 keys:

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

https://backpack.tf/item/5435369346

21 keys:

https://i.imgur.com/spbPmj0.png

https://backpack.tf/item/3668380248

50 keys:

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

https://backpack.tf/item/2729908478

Sales at:

4 (2x)

5.5

6.5

7.5

Excluding 6.5 and 7.5 due to the 2 for sale at 6.

Going with 4-5.5.