Reddit mentions of How to Make Money on Mars: The Economic Case for Exploring Mars and Beyond

Some resources may be easier to harvest on Mars then Earth (either due to political or ecological reasons or are just physically easier to access) and martian resources should be easier to use for exploration beyond the inner planets. But short of platinum group metals its hard to imagine trade with Earth on minerals would be very strong unless there is a dramatic innovation​ in cost of transit to and from Mars.

But Mars doesn't have to only generate income via trade of resources with Earth. Technology and information trade will have value as well. The pressures of suriving on Mars will force a technologic boom in sustainable living and non fossile fuel based energy. Such technologies will obviously have value on Earth as it would the rest of the solar system. And these ideas csn be transmitted at the speed of light. Also spaceship technology will improve as they have a drive to invent better then just workable for faster and more enjoyable transit to and from Mars.

And probably a dozen other things I can't​ think of at the moment. I read a short ebook on this once, its worth a scan if you are interested in these kinds of things.