The Commercial Spaceflight Federation, an alliance of businesses and organizations, just endorsed the President's current vision for space exploration. That's not the interesting part, at least to me. The interesting part is the Federation's stated goals: "to promote the development of commercial human spaceflight, pursue ever higher levels of safety, and share best practices and expertise throughout the industry." Shouldn't economic self-sustainability trump everything else? If space flight pays for itself, it will continue. If it's an expensive amusement park ride, it won't.
Has anyone done even 30,000'-view type proposals for commercially viable space operations? Zero-G manufacturing, towing in an asteroid for iridium? Yes, it's neater to send people instead of computers into space, but people break more easily and cost more money. And yes, the ultimate far-future goal should, as the Spaceflight Federation states, to get humans into space, and to settle bodies besides the Earth as insurance against existential threats, but it has to begin with economically sustainable activity.
[Added later: Tyler Cowen and Charles Stross both try to answer what the smallest population is that could keep us as wealthy as we are. Cowen thinks that even a world with a billion residents would be 15% poorer (presumably on a per capita basis) than our current world. In other words, a billion isn't enough to sustain our current standard of living even on a planet that has an atmosphere and water. Can we really expect that substantially fewer people can sustain just the bare essentials for survival on Mars, which would include technologically complex equipment? Until you get that basic sustenance, other colonies are necessarily dependent on Earth for their survival. Not surprising, since we haven't even sustainably colonized Antarctica yet, even with air and water.]