Wanted to save you a click and write what record was actually set. Article doesn’t mention it.
It’s not most expensive SW prop

a miniature X-Wing starfighter from “A New Hope” sold for over $2.3 million in June

So it might be most expensive firearm prop, but we are left in the dark…
The article’s author also doesn’t know anything about props:

The original prop gun was modeled after the Mauser C96 broom handle pistol used by Frank Sinatra in “Naked Runner,” and was built by “Star Wars” set decorator Roger Christian and armorer Carl Schmidt. It has gone on to be dubbed the “Hero” prop and is considered one of the most recognizable weapons in cinema history.

A hero prop is just the name for a prop that’s seen in close-ups or is important.