According to Ignoramus5533 :
I came into possession of a few used 400-600A circuit breakers. Each
is the size of a milk jug. While researching prices on them and such
(they sell for $150-200 on ebay), I learned that some cost many
thousands of dollars new. I saw numbers from $2,500 and higher.

I am curious just what makes them so expensive. Do they have any
precious materials inside? Or what?

They're basically relatively low demand things used
commercially/industrially, and they're much more rugged than
their residential counterparts.

It takes good design to reliably interrupt a 400-600A circuit without
blowing up the neighborhood. Especially at higher voltages.

If you think that's bad, you should see the breakers they have
to use with higher ampacities and voltages. "air blast arc
suppression" etc. I'm glad we don't have to do that at a mere
15A ;-)
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.