I work Exploration, so I'm biased, but here (again) are my totally personal and absolutely unofficial opinions that represent no one but me:
Why explore Mars? My answer is simple and potentially lucrative. One of the primary goals of NASA, and a worthwhile one, is the search for extraterrestrial life. Mars is perhaps the easiest place to look that has a reasonable chance for success. I'm not implying we'll find little green men or Marvin the Martian, but the possibility that there's some kind of extremophilic bacteria or fungus or something of that caliber is a distinct one.
So who's excited about Martian bacteria? Well, you should be. Now that we have genotyping and transgenics developed, such an organism could provide a huge boost to microbiology. A large fraction of current biotech -- and a hundred billion dollars in business and research -- can be traced to a lone extremophile,
Thermus aquaticus, that was found in the volcanic vents of the deep sea. Anything capable of surviving on Mars would have adapted to low temperatures, high salinity, a low-pressure reducing atmosphere, metal chemistry, and probably high radiation depending on exactly where it lives.
It is also possible that any organism there doesn't fit our standard model of life at all, but that would be even more exciting, and potentially even more useful.
Martian geology can also provide insight into our own planet's evolution. As we now know, Mars had abundant surface water, and by inference a more substantial atmosphere. This is not well understood at all. Furthermore, here on Earth, we have recovered a few pieces of Mars (over 30), as meteorites that were thrown off and found their way into our gravity well. The reverse will also be true. The MER mission has
already found at least one meteorite (though not from Earth) on the Martian surface -- it practically landed on the thing. This particular search is arguably better done on the Moon, but there are reasons why Mars is easier than the Moon, just as there are other reasons why Moon is easier than Mars. As we grapple with questions like AGW, and prepare to spend an estimable fraction of our entire output as a species to either react to or ignore it, what would it be worth to have better modeling tools and additional evidence of the past record?
Colonization is a harder sell. Long-duration missions for the purpose of exploration are defensible through the above, but this does not imply or require a self-sufficient colony. Unfortunately colonies are not and probably will never be the kind of places that ordinary people travel -- there just isn't enough energy lying around to do that. I've argued elsewhere (and professionally) that when we ultimately do colonize other planets, the colonists are not likely to be truly human at all, but instead an engineered species with innate resistance to different environmental hazards. Radiation is the big one. We do not have an answer for it at this time, period.
Mars is also not the best candidate, as always in my own humble opinion, for a colony. Low orbit is. Next best would be the asteroids. Moon or Mars are next, and which is easier depends primarily on propulsion technology -- Moon is closer, but Mars is easier to land on provided you don't mind waiting longer to transfer.
Regarding costs, thus far the costs of
Mars exploration have been negligible -- approximately $25B in current dollars, counting everything from Mariner 4 to MSL, spread between several countries. A crewed mission to Mars will, of course, be remarkably expensive, but this is merely a discussion in priorities. What would you rather have scientists work on? There is a business case to be made here, it merely takes vision and will that lasts a couple of decades. Piece of cake.
