Sale 1:

Sold for 13 keys + 5 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1515456000/1515542400

Buyer: https://backpack.tf/profiles/76561198187011719#!/compare/1515456000/1515542400

X = 13

Sale 2:

Sold for 10 keys

Seller: https://backpack.tf/profiles/76561198796496259#!/compare/1515888000/1515974400

Buyer: https://backpack.tf/profiles/76561197976392732#!/compare/1515888000/1515974400

X = 10

Sale 3:

Sold for a tomislav signed by the famous csgo-bolt.ru (easily 1000+)

Seller: https://backpack.tf/profiles/76561198187011719#!/compare/1515801600/1515974400

Buyer: https://backpack.tf/profiles/76561198796496259#!/compare/1515456000/1515801600

X = at least 1000

Sale 4:

Bulk for ’72 flip

Seller: https://backpack.tf/profiles/76561198072745436#!/compare/1512345600/1516406400

Buyer: https://backpack.tf/profiles/76561198045122249#!/compare/1516320000/1516406400

Sale 5:

Sold for Showstopper Oblooterated

Seller: https://backpack.tf/profiles/76561198045122249#!/compare/1516838400/1516924800

Buyer: https://backpack.tf/profiles/76561198444456236#!/compare/1516838400/1516924800

Can use on the Obloot, or not at all.

Sale 6:

Sold for SGG Disco

Seller: https://backpack.tf/profiles/76561198444456236#!/compare/1517011200/1517097600

Buyer: https://backpack.tf/profiles/76561197976392732#!/compare/1517011200/1517097600

Same story as the obloot

Sellers:

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

12 keys:

https://backpack.tf/item/4174505766

https://backpack.tf/item/3295537598

18 keys:

https://backpack.tf/item/2851565752

Sales at 10, 13 and 1000+.

Assuming 1000+ is the outlier.

Going with 10-13.