I think this is a good long-term goal, but one thing that's missing from this analysis is the urgency of the thing. Right now, we've never even put one person on Mars, and keeping any significant Martian population alive would be a considerable expense. The cost in human labor and materials would certainly have to be diverted from other enterprises, as is the case with all economic actions.

Given that we are on an exponential growth curve both in terms of technology and wealth, it seems likely to me that by the natural course of things 100 years from now maintaining an extraplanetary base on Mars would be much cheaper and easier, and would disrupt our growth rates much less, if at all. It may be that by acting now, it would take us 200 years to create a self-sustaining population on Mars, whereas if we wait 100 years, we'll be able to get something up and running in just an additional 50 years, thus better insulating us from disaster (consider that until the Mars colony is completely self-sufficient, it will likely be wiped out by any disaster on Earth, even disasters that aren't a real threat to our species, as long as we don't have enough surplus wealth to sustain an expensive Mars colony).