Using last month.

Sales 1-3:

Sold for $59.99, $49.95 and $16 on Marketplace.tf

https://marketplace.tf/items/1108;5;u3007

$16 is gonna be an outlier, excluding.

59.99/1.97 = ~30.45

49.95/1.96 = ~25.48

X = 30

X = 25

Sale 4:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1524441600/1524528000

Buyer: https://backpack.tf/profiles/76561198327709625#!/compare/1524528000/1524787200

Untraceable (apart from key histories), bot.tf confirms: https://i.imgur.com/5plPRPA.png

https://backpack.tf/item/6800567338

https://backpack.tf/item/6800567362

https://backpack.tf/item/6800567371

https://backpack.tf/item/6800567388

https://backpack.tf/item/6800567414

https://backpack.tf/item/6800567438

https://backpack.tf/item/6800567454

https://backpack.tf/item/6800567467

https://backpack.tf/item/6800567489

https://backpack.tf/item/6800567509

https://backpack.tf/item/6800567538

https://backpack.tf/item/6800567560

https://backpack.tf/item/6800567579

https://backpack.tf/item/6800567597

https://backpack.tf/item/6800567623

Buyer doesn’t appear in the key histories because their backpack wasn’t scanned when they had them. They would’ve bought the keys from marketplace.tf shortly before buying the taunt.

X = 15

Sale 5:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1523836800/1523923200

Buyer: https://backpack.tf/profiles/76561198046783657#!/compare/1523232000/1523923200

Untraceable, but bot.tf confirms: https://i.imgur.com/5plPRPA.png

X = 15

Sale 6:

Sold with ~4 keys in items for Cloud 9 Panama (18.5)

Seller: https://backpack.tf/profiles/76561198074184778#!/compare/1525651200/1525737600

Buyer: https://backpack.tf/profiles/76561197994697258#!/compare/1525651200/1525737600

X = 15

Sale 7:

Quicksold for 13 keys

Seller: https://backpack.tf/profiles/76561197972383519#!/compare/1525132800/1525219200

Buyer: https://backpack.tf/profiles/76561198113233365#!/compare/1525132800/1525219200

X = 13

Sale 8:

Quicksold for 12 keys

Seller: https://backpack.tf/profiles/76561198002757046#!/compare/1525996800/1526083200

Buyer: https://backpack.tf/profiles/76561198050052801#!/compare/1525996800/1526083200

X = 12

Sale 9:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198332925604#!/compare/1525564800/1525651200

Buyer: https://backpack.tf/profiles/76561198074184778#!/compare/1525564800/1525651200

X = 10

Sellers:

24 keys:

https://i.imgur.com/R22jMGB.png

https://backpack.tf/item/2958931092

25 keys:

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

https://backpack.tf/item/5181266811

https://backpack.tf/item/2807409439

30 keys:

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

https://backpack.tf/item/6478071516

50 keys:

https://i.imgur.com/VwR4fw3.png

https://backpack.tf/item/5135003064

Sales at:

10

12

13

15 (3x)

25

30

12 and 13 quicksold, excluding them and everything lower.

The marketplace sales looks like large outliers. Excluding them.

Going with 15 flat.