The #1 export from Mars will certainly be "data about Mars, in digital form" for a long time. Maybe some software directly developed for that which then turns out to have other uses. Art would probably be part of this.

People might want to go to Mars for non-economic reasons and then continue to do software development, too. (I'd happily pay $500k to go to Mars if it looked like doing so wouldn't rule out life extension technology, either because life extension was stalled or because it would also be available on Mars. The only thing I wouldn't give up for Mars is a good chance at immortality.)