"For the sci-fi piece then what I need is some reason why anyone would want to set up a sheltered colony on Mars - when I had the possibility of something Earthlike and habitable, that was sufficient motivation"

I'm not sure exactly what you are saying, but...

"I can't get exploration in and of itself to work as a justification for the sheltered colony - I need some kind of economic good -"

Ok, first of all no one (not even insanely foaming Martians) is talking about terraforming Mars in the near term. Some Mars fanactics justify exploring Mars because "someday" it could be terraformed.
As far as "economic good", I think that this is the major reason we should be doing any space exploration- this includes exploration with robots or even telescopes. "Pure" research never gets very much money from societies nor do I think it should. Now, governmental science should have "some efforts" toward more distance horizon before expectation of any "pay back"- but it should be mostly focused in the near term.