Used last 3 months, as we didn't have many sales within the last 2 months.

Sale 1:

Sold for 15 keys

https://backpack.tf/profiles/76561198042844970#!/compare/1704672000/1705276800

https://backpack.tf/profiles/76561198117597416#!/compare/1704499200/1704672000

https://backpack.tf/item/6192007129

= 15 keys

Sale 2: (+2 months)

Sold for 15 keys

https://backpack.tf/profiles/76561198113190503#!/compare/1698364800/1698451200

https://backpack.tf/profiles/76561197997843710#!/compare/1698451200/1698624000

https://backpack.tf/item/12558930488

= 15 keys

Sale 3: (+2 months)

Sold for 15 keys

https://backpack.tf/profiles/76561198453530349#!/compare/1698278400/1698364800

https://backpack.tf/profiles/76561198092643245#!/compare/1698192000/1698364800

https://backpack.tf/item/9130937631

= 15 keys

Sale 4:

Sold to a bot for 14 keys + negligible metal

https://backpack.tf/profiles/76561198046264916#!/compare/1703289600/1703462400

https://backpack.tf/profiles/76561198850274253#!/compare/1703376000/1703462400

https://backpack.tf/item/4459559972

= 14 keys

Other Sales:

#1

https://backpack.tf/profiles/76561199106461781#!/compare/1705017600/1705363200

https://backpack.tf/profiles/76561199059800460#!/compare/1704931200/1705017600

https://backpack.tf/item/12537719854

Untraceable

#2

https://backpack.tf/profiles/76561198415964653#!/compare/1702166400/1702252800

https://backpack.tf/profiles/76561198453530349#!/compare/1702080000/1702166400

https://backpack.tf/item/6272249915

Untraceable

#3

https://backpack.tf/profiles/76561198453530349#!/compare/1704499200/1704585600

https://backpack.tf/profiles/76561198141334389#!/compare/1704412800/1704585600

https://backpack.tf/item/6272249915

Untraceable

#4

https://backpack.tf/profiles/76561198051839199#!/compare/1701561600/1702598400

https://backpack.tf/profiles/76561199072654974#!/compare/1702512000/1702598400

https://backpack.tf/item/4496986281

Untraceable backpack buy

#5

https://backpack.tf/profiles/76561199072654974#!/compare/1703376000/1703462400

https://backpack.tf/profiles/76561198171727325#!/compare/1702771200/1703548800

https://backpack.tf/item/4496986281

Untraceable

#6

https://backpack.tf/profiles/76561199242468782#!/compare/1702425600/1703548800

https://backpack.tf/profiles/76561198046264916#!/compare/1702425600/1702684800

https://backpack.tf/item/4459559972

Sold for Bubbling Brain Bucket

#7

https://backpack.tf/profiles/76561198075333342#!/compare/1703721600/1703808000

https://backpack.tf/profiles/76561198057412106#!/compare/1703721600/1703808000

https://backpack.tf/item/7227241865

Backpack buy

https://backpack.tf/item/9768962001 - mannco

https://backpack.tf/item/6895611908 - mannco

Marketplace Sales:

https://marketplace.tf/items/tf2/1162;5;u3003

$35.99, November 10th (+2 months)

https://marketplace.tf/items/tf2/5021;6

Keys were $1.74 on November 10th

35.99/1.74 = ~20.7, rounds to 21

Sellers:

https://imgur.com/a/zy5Ed1L

https://backpack.tf/item/14052677286 - 17 keys

https://backpack.tf/item/4459559972 - ~17.5 keys

https://backpack.tf/item/7227241865 - marketplace

https://backpack.tf/item/9213018674 - marketplace

Sales at:

14

15 (3x)

21

21 is obviously way too high here, excluding 21.

14 is a lone sale to a bot. Excluding 14.

Taking 15 flat.