Using last month

Sales 1 & 2:

Sold for $32.99 and $27.02 on Marketplace.tf

https://marketplace.tf/items/1162;5;u3008

32.99/1.96 = ~16.8

27.02/1.97 = ~13.72

X = 17

X = 14

Sale 3:

Sold for 15 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523059200/1523145600

Buyer: https://backpack.tf/profiles/76561198232157836#!/compare/1523059200/1523145600

Bot.tf confirms: https://i.imgur.com/QY2A19o.png

X = 16

Sale 4:

Sold for Killstreak Aus FaN (10)

Seller: https://backpack.tf/profiles/76561198820686501#!/compare/1523318400/1523577600

Buyer: https://backpack.tf/profiles/76561198017911489#!/compare/1523232000/1523318400

X = 10

Sale 5:

Bulk for Insomnia HKC

Seller: https://backpack.tf/profiles/76561198140010255#!/compare/1522540800/1522972800

Buyer: https://backpack.tf/profiles/76561198305458404#!/compare/1522454400/1522972800

Sellers:

55 keys:

https://i.imgur.com/lr7dyHP.png

https://backpack.tf/item/6459338369

Sales at:

10

14

16

17

10 is an outlier, excluding 10.

Going with 14-17.