I'm glad Zubrin brings up the economics of Mars colonization, since that's easy to neglect. Once you're there, how do you make money?

Mining might be lucrative if Mars has gold or platinum or some valuable mineral that's easy to extract and worth more than its shipping cost back to Earth.

I'm skeptical that patents are going to be a major export. Inhabitants of Mars presumably will have better uses of their time than filing patents, and they would be competing against Earth-bound innovators against whom they don't have any particular advantage other than necessity. (Ideally, Mars wouldn't itself even be subject to patent law.)

Space Tourism will be a thing unless there's some explicit policy to prevent wealthy thrill-seekers from going to Mars if they don't plan on doing any actual work while they're there.

Science and exploration might be valuable professions. Like, if people on Earth put bids on locations, saying "I'll give you a thousand dollars if you drive your rover out to this location and take a few pictures and pick up some rock samples". Mining companies might be especially interested, but so would Earth-bound scientists who just want to know more about Mars.

Real-estate speculation might be another cottage industry. Developers are going to want to establish homesteads in valuable locations, and then they can sell adjacent lots to newcomers. (This assumes some kind of sane framework for land ownership. Hopefully such a thing will strike a sensible balance between being able to claim "dibs" on entire landscapes vs not having property rights at all.) As long as the population of Mars is growing, this could be a lucrative profession.