Elon Musk has said that there is no way a Mars colony can export anything that could pay for the cost of exports to Earth. He agrees that a colony needs to pay for imports, and proposes to support the Mars colony by licensing inventions and other intellectual property produced by the colonists on Mars to Earth.

"I don't think it's going to be economical to mine things on Mars and then transport them back to Earth because the transport costs would overwhelm the value of whatever you mined, but there will likely be a lot of mining on Mars that's useful for a Mars base, but it's unlikely to be transferred back to Earth. I think the economic exchange between a Mars base and Earth would be mostly in the form of intellectual property"

Elon Musk interview on the future of energy and transport - and more quotes like this.

Robert Zubrin covers this in more detail:

"Another alternative is that Mars could pay for itself by transporting back ideas. Just as the labor shortage prevalent in colonial and 19th century America drove the creation of Yankee Ingenuity's flood of inventions, so the conditions of extreme labor shortage combined with a technological culture and the unacceptability of impractical legislative constraints against innovation will tend to drive Martian ingenuity to produce wave after wave of invention in energy production, automation and robotics, biotechnology, and other areas. These inventions, licensed on Earth, could finance Mars even as they revolutionize and advance terrestrial living standards as forcefully as 19th Century American invention changed Europe and ultimately the rest of the world as well."

I think there may be other possibilities of exports, but the case is much harder to make for Mars than for the Moon., because so much delta v is needed to export materials from the surface.

In the case of the Moon, then using Hoyt’s cislunar tether system, which would not be that hard to set up, you can export to Earth LEO or back to Earth itself with almost no delta v. It’s hard to beat that. So surely the Moon would undercut Mars for anything that can be produced on the Moon.

I think we should go to the Moon first anyway, and go slow with Mars for planetary protection issues. The Moon is far more interesting than most realize. I think if humans go to Mars they should stay in orbit for now and explore the surface with avatars controlled by close to real time telepresence from orbit or from its moons. That’s lower cost, safer and I think it would get much more exploration too for the same cost without risking introducing Earth microbes to Mars to confuse the searches for indigenous Mars life or indeed perhaps make it extinct. Either that or explore from Earth

It is possible for Deimos to export to Earth, less delta v than from the surface, but again it might well be undercut by the Moon.There are a few ideas though of things that could be exported from Mars.

Anyway this is my section about the commercial value of Mars in my “Case for Moon First” book: Case For Moon First - Commercial value of Mars