Using sales in the last 30 days.

Sales 1 & 2:

Sold for $60.21 and $40 on Marketplace.tf

https://marketplace.tf/items/477;5;u3003

https://backpack.tf/item/3753724858 (might be one of them)

https://backpack.tf/item/6115275046

60.21/1.98 = ~30.4

40/1.98 = ~20.2

X = 20 & 30

Sale 3:

Sold for 20 keys + 18 ref

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1521072000/1521158400

Buyer: https://backpack.tf/profiles/76561198054678875#!/compare/1521072000/1521158400

Confirmed by bot.tf: https://i.imgur.com/iKf4sZi.png

X = 21

Sale 4:

Sold for 18 keys

Seller: https://backpack.tf/profiles/76561198344933095#!/compare/1522368000/1522454400

Buyer: https://backpack.tf/profiles/76561198062710038#!/compare/1522108800/1522454400

https://i.imgur.com/iKf4sZi.png - Bot.tf confirms

X = 18

Sale 5:

Sold for Showstopper Didg (10)

Seller: https://backpack.tf/profiles/76561198048511979#!/compare/1520035200/1520121600

Buyer: https://backpack.tf/profiles/76561198429587621#!/compare/1520035200/1520121600

X = 10

Sale 6:

Quicksold for 10 keys

Seller: https://backpack.tf/profiles/76561198033694310#!/compare/1520121600/1520208000

Buyer: https://backpack.tf/profiles/76561198097271466#!/compare/1520121600/1520208000

X = 10

Sale 7:

Sold for 9 keys

Seller: https://backpack.tf/profiles/76561198118343427#!/compare/1520553600/1520640000

Buyer: https://backpack.tf/profiles/76561198382034007#!/compare/1520553600/1520640000

X = 9

Sale 8:

Quicksold with ~1 key in items for 9 keys

Seller: https://backpack.tf/profiles/76561198429587621#!/compare/1520121600/1520208000

Buyer: https://backpack.tf/profiles/76561198120680312#!/compare/1520035200/1520121600

X = 8

Sale 9:

Sold for Electrostatic Waxy Wayfinder

Seller: https://backpack.tf/profiles/76561198405692100#!/compare/1520380800/1520467200

Buyer: https://backpack.tf/profiles/76561197964171516#!/compare/1520294400/1520380800

Can be used on the Wayfinder.

Bot.tf shows another one sold for 10 keys + 11 ref recently, but I couldn’t find the item’s history:

https://i.imgur.com/iKf4sZi.png

Sellers:

25 keys:

https://i.imgur.com/DMOFfb8.png

https://backpack.tf/item/2688056539

50 keys:

https://i.imgur.com/dQqb4wt.png

https://backpack.tf/item/4029117084

Sales at:

8

9

10 (3x)

18

20

21

30

Buyer at 13 keys: https://i.imgur.com/ZIz7Njd.png

Excluding sales below 13.

30 is an outlier, excluding 30.

Going with 18-21.