Sale 1:

Sold for 4 keys + 14.77 ref (buyer's end)

https://backpack.tf/profiles/76561199085308746#!/compare/1703635200/1703721600

https://backpack.tf/profiles/76561199115825381#!/compare/1703635200/1703721600

https://backpack.tf/item/4742694518

= 4.25 keys

Sale 2:

Sold to a bot for 4 keys + 16 ref

https://backpack.tf/profiles/76561199115825381#!/compare/1703894400/1703980800 - Near ID: 13988982420

https://backpack.tf/profiles/76561198311498888#!/compare/1703894400/1703980800

https://backpack.tf/item/4742694518

= 4.25 keys

Other Sales:

#1

https://backpack.tf/profiles/76561199059328260#!/compare/1702944000/1704758400

https://backpack.tf/profiles/76561198850274253#!/compare/1702857600/1702944000

https://backpack.tf/item/7103637144

Untraceable

#2

https://backpack.tf/profiles/76561199404812646#!/compare/1702598400/1703462400

https://backpack.tf/profiles/76561199085308746#!/compare/1703376000/1703462400

https://backpack.tf/item/4742694518

Untraceable. 54.33 ref matches.

#3

https://backpack.tf/profiles/76561198142735602#!/compare/1704672000/1705622400

https://backpack.tf/profiles/76561199043177646#!/compare/1705536000/1705622400

https://backpack.tf/item/4598507189

Untraceable

#4

https://backpack.tf/profiles/76561197997138838#!/compare/1702080000/1702166400

https://backpack.tf/profiles/76561199110809353#!/compare/1702080000/1702166400

https://backpack.tf/item/3971265490

Bulk

https://backpack.tf/item/14094310727 - gambled

https://backpack.tf/item/10249552958 - traded between quicksell.store bots

Sellers:

https://imgur.com/a/FE8zPfK

https://backpack.tf/item/4742694518 - ~4.75 keys

https://backpack.tf/item/8138611834 - ~5 keys

https://backpack.tf/item/3901687238 - ~5 keys

https://backpack.tf/item/4598507189 - ~5.45 keys

Sales at:

4.25 keys (2x)

https://gladiator.tf/sales?item=Holy%20Grail%20Taunt:%20The%20Box%20Trot&at=2023-12-22T14:14:00.912Z

This unconfirmed gladiator sale at ~4.2 keys supports the price.

Taking 4.25 keys.