A couple of posts below, Ken lays out a good argument for developing industry on Mars.
The only point I disagree with is the assumption that there's no market for exporting products from Mars.
An export industry is an industry, and there is, or soon will be, a very viable market; consumables.
Let's look at fuel depots in LEO for just one example; they're going to happen, and fairly soon. So, how does one get fuel to the fuel depot? Currently, plans are to launch it from Earth. This seems logical, because low earth orbit is only around 200 miles from Earth's surface. However, when it comes to spaceflight, the number that matters most is propulsive delta-V, not distance. To get to LEO, you have to accelerate your payload by around 9.2 kilometers per second (8 kps for orbital speed, and about 1.1 to 1.2 for gravity and aerodynamic losses).
Therefor, if you have a fuel depot for, say, methane and oxygen, or hydrogen and oxygen, getting it to LEO from Earth is a hard hill to climb. So, why not get it from Mars? That sounds preposterous because Mars is a long way from Leo, but that's in distance, not in Delta-V, and Delta-V is what counts.
Launching from the surface of Mars to low Martian orbit takes 4.1 kps, a lot less than the 9.2 from Earth to Leo. But, how do you get from low Mars orbit to LEO? Part of that is propulsive Delta/v - 2.3 kps to get you on an earth-transfer trajectory. So, your total propulsive delta-V? 6.4 kps. From Earth transfer, you can aerocapture (no shielding required) into LEO. So, your fuel arrives in LEO having needed 6.4 kps propulsive Delta-V compared to 9.2 from Earth. That's quite a savings, but it's only a small part of the benefits. The rest is in launch architecture.
Take the F9 upper stage as an example. If you had it on the surface of Mars, not only could it do single stage to orbit with what, on earth, is its max payload, it could do Mars surface to earth transfer. (Falcon 9 stage sep occurs around mach 6, so 1.7 kps, and though the first stage has dealt with about half the gravity losses, the second stage has to supply around 6.8kps)
What this means in practice is that your Mars launch vehicle can be single stage to orbit - far, far cheaper and easier than staging, because it eliminates the need for the 90% of the vehicle you discard at staging (and even with reusability, this cost dynamic applies. a 747 and a Lear Jet are both reusable, but the 747 costs one hell of a lot more to buy and operate.)
With ISRU on Mars, you could easily produce methane and oxygen, launch them to low Mars orbit, where you use them to fill up a supercold transfer tank. (you avoid boil off losses by keeping the tank very, very cold, easy to do in space; you have a layer of aluminum foil a few feet from the tank, between it and the sun). Once the tanker is filled, a small rocket motor using some of that fuel boosts it through the trans-earth burn.
Hydrogen/oxygen is a bit trickier due to the temperature demands for Hydrogen, plus the tankage size needed. It's probably be more efficient to ship it as water, and electrolyze it as needed in earth orbit.
And speaking of water, that, too, is a commodity that'll be needed as space infrastructure grows. And that gets us to food, which is already a significant amount of upmass for ISS, and the demand for it will rise as we see more human occupation of LEO as well as further out.
So, given all of the above, we have an economic model for a major export industry on Mars. However, there's an even bigger economic factor, one that harks back to the birth of Transatlantic trade. The early colonies in North America were founded with people and supplies from Europe, and initially, the only value returned was that of the land in the New World. However, the ships delivering supplies were going back empty, and that's free cargo capacity, then as now. Back then, cash crops such as tobacco began to fill those empty holds. The same dynamic applies to a Mars colony; the vehicles delivering colonists and supplies to Mars will be going back to earth empty at first, so that's basically free cargo capacity. For a colony on Mars, you'll already have ISRU production of water, fuel, etc, and for vehicles like MCT, which is supposed to be reusable (and thus must return to earth), you'll have 100 tons cargo capacity to LEO. Whether MCT is able to land on Mars (I personally doubt it) of is a space-only craft, you'll have some kind of Mars-ascent system. The capability is inherent in the design concept of the raptor-powered heavy lifter; an upper stage of that reusable launch vehicle would have the inherent ability to land on Mars, refuel, and do either single stage to Mars orbit, or, single stage from Mars surface to Leo. So, to use the early transatlantic trade analogy, you'll already have the empty holds returning to Earth, so why not use them for an export industry?
Later, as the Mars colony's capabilities expand, if they're able to build pressurized habitats for Mars (they'll have to be able to do that) they can just as easily build them for space, and put them where they are needed for far, far less cost than launching them from earth. It might also be worth their while to build fuel depots (which are largely just large tanks), fill them in Mars orbit, and send them earthward - because one of the many advantages of launching from Mars is you're not limited by the diameter of the launch vehicle or payload shroud (restrictions imposed by earth's atmosphere). This of course applies to habitat design as well; if somebody wants, say, a 100 foot diameter sphere, or a segment of a large wheel, that's easy to launch from Mars but impossible from Earth, due to the atmosphere.