Using last month.

Sale 1:

Sold for $37.80 on Marketplace.tf

https://marketplace.tf/items/30672;5;u3010

https://backpack.tf/item/4304641767

No outpost

37.80/1.98 = ~19.1

X = 19

Sale 2:

Sold for 16 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523664000/1523750400

Buyer: https://backpack.tf/profiles/76561198005557902#!/compare/1523577600/1523750400

Bot.tf confirms: https://i.imgur.com/fRK9Wm1.png

X = 16

Sale 3:

Sold for 13 keys (ID’s match)

Seller: https://backpack.tf/profiles/76561198442764620#!/compare/1523318400/1523404800

Buyer: https://backpack.tf/profiles/76561197980575764#!/compare/1521849600/1523404800

Bot.tf confirms: https://i.imgur.com/fRK9Wm1.png

X = 13

Sellers:

13 keys:

https://i.imgur.com/55vZHg0.png

https://backpack.tf/item/4752295794

14 keys:

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

https://backpack.tf/item/4606658452

19 keys:

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

https://backpack.tf/item/6605876207

24 keys:

https://i.imgur.com/5hWbwKl.png

https://backpack.tf/item/4273290662

25 keys:

https://i.imgur.com/LFfbIA3.png

https://backpack.tf/item/4274912647

Offers:

http://www.tf2outpost.com/trade/29635816

And another on marketplace.tf for $22:

https://marketplace.tf/items/30672;5;u3010 – was listed while I was working on the suggestion.

Sales at:

13

16

19

Sticking with the pure sales here.

Going with 13-16.