Marketplace Sales:

https://marketplace.tf/items/1113;5;u3013

https://manic.tf/keyprice/?at=2019-02-14 - $50 / $1.8 = 27.78 keys

https://backpack.tf/item/2911887153 - couldn’t get a b/o

X = 28

Other Sales:

https://backpack.tf/item/5203077293 (#1)

https://backpack.tf/item/3644875694 (#2)

https://backpack.tf/item/5235485636 - scrap/marketplace/tf2mart/tradeit

#1

Untraceable

https://backpack.tf/profiles/76561198005427195#!/compare/1547856000/1548028800

https://backpack.tf/profiles/76561198201305150#!/compare/1546560000/1548547200

#2

Bulk for Scorching Batter’s Helmet

https://backpack.tf/profiles/76561198823826881#!/compare/1548806400/1548892800

https://backpack.tf/profiles/76561198073438708#!/compare/1548806400/1548892800

The 28 key marketplace sale is countered by the two sellers on marketplace for $5 & $10 cheaper, as well as the unsold on the classifieds for 21:

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

https://backpack.tf/item/3644875694 - 21 keys

https://marketplace.tf/items/1113;5;u3013

https://backpack.tf/item/2911887153 - ~23 keys

https://backpack.tf/item/5235485636 - 25 keys

Dropping with unsolds.

Seller at 21 is 10 days unsold, and has owned it for a bit more than 2 months.

Going with 21 flat.