Interesting discussion, but I still scratch my head about what makes deep-space commerce "profitable" or desirable. I can understand if some uber-valuable resource is only found on certain worlds/asteroids, like dilithium. But just general ores, and other substances? Doesn't make sense to me.
What, exactly, makes them profitable in MODERN times?
I think that spaceflight and space exploration have become cheap enough to be comparable to their terrestrial counterparts by that point, so similar economics apply. You're going to have miners extracting gold, iron, silver, platinum, latinum, copper, anything else that can't be easily replicated.
The thing that gets me is that there has to be a sustainable infrastructure of space vessels (not just robot cargo ships, but also "starship protection") in order to make trade worthwhile and secure. That means not only do you have to pay for expeditions (the building of ships, trained astronauts, trained colony specialists, equipment, supplies, maintenance/repairs, etc.) but you also have to pay for "starship protection" even once the colony has established itself. All of this sounds very, very expensive to build and maintain.
Which, again, is already the case in modern maritime considerations. I don't see it being that much more expensive in the Star Trek universe than that, especially since the payoff is potentially much greater.