Sale 1:

Sold for 19 keys

Seller: https://backpack.tf/profiles/76561198116258680#!/compare/1517011200/1517097600

Buyer: https://backpack.tf/profiles/76561198196894698#!/compare/1517011200/1517097600

X = 19

Sale 2:

Sold to a bot for 16 keys

Seller: https://backpack.tf/profiles/76561198085628805#!/compare/1514505600/1514592000

Buyer: https://backpack.tf/profiles/76561198344933095#!/compare/1514505600/1514592000

X = 16

This sale occurred just over a month ago, and the market has changed since then, so I won’t be excluding the sale at 15 due to this one going to a bot.

Sale 3:

Sold for 15 keys

Seller: https://backpack.tf/profiles/76561198186770499#!/compare/1515801600/1515888000

Buyer: https://backpack.tf/profiles/76561198116258680#!/compare/1515801600/1515888000

X = 15

Sale 4:

Sold with a key for Australium Stickybomb Launcher (~12)

Seller: https://backpack.tf/profiles/76561198326093863#!/compare/1515456000/1515542400

Buyer: https://backpack.tf/profiles/76561198186770499#!/compare/1515456000/1515542400

X = 11

Sale 5:

Bulk for Scorching Noh Mercy

Seller: https://backpack.tf/profiles/76561198112099979#!/compare/1513555200/1513728000

Buyer: https://backpack.tf/profiles/76561198085628805#!/compare/1513641600/1513728000

Sellers:

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

17 keys:

https://backpack.tf/item/4072698428

18 keys:

https://backpack.tf/item/3126862620

55 keys:

https://backpack.tf/item/3648292468

Sales at:

11

15

16

19

11 is an outlier, excluding 11.

Going with 15-19.