Using the Drill here.

https://backpack.tf/item/800258834

Sale 1) Sold + 1 Key for a Double Spelled Strange Pro KS Rocket Launcher (17.5 Keys)

https://backpack.tf/profiles/76561198122096342#!/compare/1564358400/1564531200

https://backpack.tf/profiles/76561198345578899#!/compare/1564272000/1564358400

17.5 - 1 = 16.5, which rounds to 17 Keys

Rocket Mini

https://backpack.tf/profiles/76561198131555957#!/compare/1563753600/1563840000 - Sold for 19 Keys

https://backpack.tf/profiles/76561198345578899#!/compare/1564185600/1564272000 - Sold for 16 Keys

Sale 2) ?

https://backpack.tf/profiles/76561198345578899#!/compare/1564358400/1564444800

https://backpack.tf/profiles/76561198060458887#!/compare/1564358400/1564444800

Sale 3) Sold + 2 Keys for an Orbiting Fire Sergeant's Drill Hat (15.5 Keys) + ~5 Keys of Items

https://backpack.tf/profiles/76561198060458887#!/compare/1564444800/1564531200

https://backpack.tf/profiles/76561198430787433#!/compare/1564444800/1564531200

https://backpack.tf/item/811841799 - Drill

15.5 + 5 - 2 = 18.5, which rounds to 19 Keys

Drill Mini

https://backpack.tf/suggestion/5d4d1b7ed381722fe43c2b8f - 15.5 Keys

https://marketplace.tf/items/tf2/152;5;u7 - Sold for $42.99 (high outlier)

https://imgur.com/a/BqetmSJ - Young Seller at 21 Keys, 4 Day Seller at 21 Keys, and a 1 Month Seller at 22 Keys

https://backpack.tf/item/880850822

https://backpack.tf/item/800258834

https://backpack.tf/item/186754604