That something of such immense value will be found out in the solar system that it will either make retrieving it with chemical rockets worthwhile, or it will generate R&D
on a much more economical propulsion system.

But I don't think the commodity will be some valuable metal or fuel. I think it will be knowledge. My hopes are on a new life form that will give birth to a whole new biochem industry, or maybe an alien artifact (perhaps an exoplanetary interstellar probe crashed into a Kuiper belt object).

I am convinced there is stuff out there of immense value that we cannot even imagine. But we'll have to find it first, and recognize its worth. That's going to take time and money.

The best bet is some microbial life with an exotic chemistry with economic potential. But there are other possibilities. For example, what if there's a black hole floating around in the asteroid belt? Or a few isolated grams of anti-matter? Or what if we find a source of some isotope which will make efficient fusion reactors (and drives!) easy and cheap to operate?

The alien artifact is admittedly a long shot, but even that would have immense value for whatever a billion year old shipwreck might have still surviving aboard as technology, or data. Can you imagine the value of the ship's log, with a record of its travels?

Space exploration is basic research. You don't know what you're looking for, you don't have anything specific in mind, but when you find it it may be infinitely more valuable than precious metals. And very often it will be the last thing you expected.