Mars differs from the Moon in that it is actually or could be made to
be permanently habitable by a large number of humans. Mars has plenty
of its own natural energy sources, such as solar and geothermal heat,
making it fine as a "lifeboat" for our own planet (although hopefully
it won't come to that). A Mars colony could without a doubt be self-
sufficient. One of my firends from graduate school is proving that
right now on Devon Island, Canada (which has roughly the same climate
as Mars: see www.marsonearth.org). As for sending industrial products
back to Earth, it's much easier and cheaper to fly from Mars to Earth
than it is the other way around. There's no reason to think that a
Mars colony could be self sufficient almost immediately and then
(after some long time) eventually profitable.