Your question is why is issue #1 of a very old and popular thing valuable?

I get that it's more valuable than Superman #200, but why 9 million dollars valuable. There's a poorer quality one at 5 million, but that also means that there's other copies in existence.

It's also not the first Superman comic, what about Action Comics #1?

Why is the Call of Cthulhu (Weird Tales, Feb 1928) about $50k?

Or in terms of characters, what makes Superman worth 3x more than MARVEL #1 or Batman?