Economic incentives include government-funded basic science, mining, tourism, real-estate speculation (i.e. claim a bunch of land while it's cheap), entertainment (sports and movies/TV shows), the ability to do things in a presumably lax regulatory environment that might not be allowed on Earth, and long-term care for those who have disabilities that would require a wheelchair in full gravity but not on Mars, and the establishment of Mars as a more convenient source of food/fuel/raw materials for any human presence in the belt or outer solar system.
A lot of that assumes an already functioning economy on Mars. We're not going to be sending anyone other than the richest people up to Mars in their old age. And for stuff like food and hydroponics you could have that on the Moon which is the natural waystation to the Asteroids. The Asteroids are going to be where all the mining is. Entertainment also assumes an already existing economy. Space tourism is the only thing that seems viable in the short term.
About all I can see is producing methane for rockets in Asteroids and being in a much shallower gravity well than Earth. That assumes having an economy established in the Asteroid belt already though, and the cost of sustaining a colony on Mars is going to be its own kind of high overhead. It may be expensive to ship fuel off of Earth, but the people to support the refinement process already exist.
At some point there will be a flip where a Mars economy becomes self-sustaining, but so far I'm not seeing it during my lifetime. My guess is that we're going to be tethered to the Earth for a lot longer.
(Basic science is also great but that's the kind of things that governments support and its not self-sustaining any more than Antarctica is -- which I suspect is closer to the model for Mars in the next 50 years).
The ability to make methane on Mars is pretty huge, though. If you're mining the asteroid belt and need to refuel your ships, it's a lot easier to ferry fuel from Mars to the belt than haul it all the way from Earth. And methane/oxygen production is likely to be one of the first things we get going on Mars because we'll want our rockets to be able to make return trips back to Earth and we really don't want to haul the extra fuel and oxygen we need all that way.
Most of the economic incentives I suggest don't really start paying off until you have at least some decent infrastructure and a population of semi-permanent residents, though. It might not happen in our lifetimes, but you never know. Once the technology exists, it's just a matter of whether someone has deep enough pockets to make human settlement of Mars a reality. Once someone has a foothold, it gets easier for others to expand on that.
Again though lifting methane off of Earth is expensive, but ships from the Asteroid belt will naturally be coming back to Earth, so refueling around the Earth or Moon doesn't go out of the way. Mars is a detour, and you have to sink the costs to sustain the colony which offsets the smaller gravity well.
I figure that any large-scale asteroid mining operation is likely to employ a fairly complicated supply chain that doesn't ship everything to or from Earth.
Earth would probably continue to manufacture high-value, complex machines like rocket engines and computers and robots. Earth would receive high-value minerals from the asteroid belt like gold or platinum or silver. Maybe nickel or copper. Iron wouldn't be worth it unless shipping costs were really low, aluminum would probably be the same. On the other hand, iron and aluminum might be worth enough on Mars to be worth shipping there. It'd be cheaper than shipping from Earth, anyways. And hauling rocket fuel/oxygen all the way from Earth really doesn't make sense if you can get it on Mars.
Basically, you'd have ships going back and forth between Earth and the belt delivering equipment and bringing back high-value metals, and you'd have another set of ships going back and forth between Mars and the belt hauling fuel and oxygen and maybe bringing back low value metals to be used on Mars. If there are people living in the belt, the ships can bring them whatever else is easier to make on Mars than in space. Maybe food.
It's kind of a weird thought, but I would suggest that a lot of the output of industrial infrastructure in space is going to be used in space rather than returned to Earth. What's the point (you might ask) if it doesn't come back to Earth? Well, people on Earth might for various reasons want to expand their foothold in space, and then once it reaches a certain critical mass those footholds in space become their own thing. Like Europeans coming to America and eventually rebuilding more-or-less what they left behind, but with some differences.
