[elihu]

If water/oxygen/carbon are readily available, I guess that would go a long ways towards making asteroids self-sufficient. On the other hand, if they're available but one can only extract a small amount at a time as a byproduct of mining, then Mars might still be pretty attractive where those things are available in inexhaustible quantities with low effort.

I'm assuming that huge amounts of methane (or other suitable fuel) and oxygen will be wanted for propellant, though maybe ion propulsion makes that less essential.