What's the planet good for, when the supplies are most economically gathered somewhere else?
Mine water ice and hydrocarbons from Saturn's moons, Jupiter's too.
Mine metals and other materials from Earth's moon and the asteroids.
Ship them where they are needed, as needed.
What's the reason to detour all the way over to Mars?
There's a reason Niven's Known Space tales never colonized Mars, and it had nothing to do with his fictitious hydrophobic Martians. It's as much of a dumping place as Venus.
I'm not arguing that the actual mining might be cheaper and easier elsewhere, especially in the Belt, where fragments of a planet and stray chunks of nickel/iron are there for the taking, but you left out shipping. Ideally, ores would be processed in the Belt, and even more ideally manufacturing will take place there as well.
The planets won't always be aligned but Mars will always be closer to the belt (as a region) than the Earth and Moon. In any endeavour, Sh*t Happens, especially early on. While Mars is only roughly half an AU closer to the belt (if in conjunction with your destination), it also has the differing orbital velocity which makes two trips around the sun to the one of the belt. Mars orbit offers the opportunity for survey vessels to have a way station moving at a differential velocity which might not only enable, but enhance exploration and evaluation of the bodies in the belt for resources.
While the distance to any given body in the Belt depends on orbital position, Mars is only half the distance from the Sun to the belt (at just over 1.5 AU), but Earth and the Moon are even less close (1AU). Currently, that .5 AU represents months of travel, and 4.3 minutes of communications lag (for anything in the EM spectrum). Add another 8.6 minutes to the closest part of the Belt--if everything is in conjunction, and it only goes up as orbital positions vary.
Unless resources are discovered on Mars which merit its own mining operations, it is unlikely that it would be much more than a transshipment point, a support base when available, and an operations base for exploration of the Belt until bases could be established in the Belt itself. Most of that activity would be in Mars orbit, rather than on the surface, simply because of the energy savings of being off planet. Add in the human factor and until you can generate artificial gravity (which could change all the equations in re drives and energy needed), humans will require some time in even partial G to function correctly, biologically.
Similar arguments were made during the space race with Von Braun's concept of an orbital station above Earth before going to the Moon. That Station could have served as an assembly point for interplanetary and Lunar ships and operations, instead, in our race with the Russians, we opted for the Moon more directly. When we were done we had some stunning photographs, a feather in our cap, and a few boxes of rocks, many of which were handed out as souvenirs. Bragging rights, but little enough to show long term for the effort besides what we learned while going and while there. I'm not discounting that knowledge, but it seemed little enough for the effort, and hardly tangible progress toward colonization or even exploitation of Space and the vast resources out there.
