Using last month

Sales 1, 2, 3 & 4:

Sold for $12 on Marketplace.tf 3 times in 1 day, and once for $19.99

https://marketplace.tf/items/1117;5;u3010

I’m pretty certain these are the histories of the items:

https://backpack.tf/profiles/76561198121830870#!/compare/1523577600/1523664000

https://backpack.tf/item/5232162598

https://backpack.tf/item/2883078466

https://backpack.tf/item/6001290150

19.99/1.97 = ~10.15

12/1.97 = ~6.09

X = 10

X = 6 (3x)

Sale 5:

Bulk for Smoking DCO

Seller: https://backpack.tf/profiles/76561198074178367#!/compare/1524700800/1524787200

Buyer: https://backpack.tf/profiles/76561198409674874#!/compare/1524614400/1524787200

Sellers:

5 keys:

https://i.imgur.com/z1QOaG1.png

https://backpack.tf/item/2991180208

10 keys:

https://i.imgur.com/Q2uzfsA.png

https://backpack.tf/item/2840922657

14 keys:

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

https://backpack.tf/item/4470131803

Offers:

http://www.tf2outpost.com/trade/29633081 - bulk

Sales at:

6 (3x)

10

10 is an outlier, excluding 10.

Seller at 5 is 10 days old, using 5 as the low-end.

Going with 5-6.