Which, to me, suggests most of the "ore freighters" of the TOS era would probably be interplanetary spacecraft, not interstellar ones. Refined materials would be shipped by boomers of the cargo service since they are more valuable, but unrefined material probably goes out over short-range robot ships. I haven't seen many references to that, although I would say that in a system like, say, Alpha Centauri that may not be such an impressive feat as you would think. Hardly. I'll again draw attention to the Janus colony, which is described as being very geologically rich to an extent that a "second Earth" could never be. More to the point, Janus is otherwise uninhabitable and isn't very attractive to anyone EXCEPT for its mineral wealth. I would speculate that "garden planets" that are very Earthlike are probably valuable for mundane reasons (e.g. realestate) in which case the residents would have to purchase almost EVERYTHING from other colonies that actually manufacture things. They'd have to pay for those products probably by work either in the service industry and other intangible goods, or by physically shipping out from the garden world and doing the hard labor in some of those other less hospitable/high value asteroid colonies. I don't know that "farming colonies" would actually be all that profitable in and of themselves, but would probably be extremely necessary for Federation planning and would probably rely almost exclusively on subsidies.