At least initially, I think ticket sales will do. With ticket prices floated for like 20 years before any go on sale, the target price is like $200,000 or so and the idea is that many people will be both willing and able to sell their Earthly possessions and go.
They will produce multiple exports from Mars that are most likely going to be scientific data and software, things that are easily transmittable back to Earth (unlike rocks etc).
I would also imagine the more distant future profit models will be similar to on Earth; people will buy and sell goods and services there, just like they do here. There will be Martians who want to go explore the rest of the solar system, so they will build that tech as well.
I could list more and maybe I will but I gotta run. There are limitless potential sustainable profit motives and economic activities that will happen on Mars; until we go and establish the colonies, we can only speculate the exact nature of them. But given that millions of people are saving their money clamoring to move to Mars ASAP, I imagine there will be millions of ideas for generating income on Mars.
Where there are problems that people have, there will be solutions that people will pay for. And there will be millions of people on Mars - so I expect millions of problems and millions of solutions.
Edit: I imagine sports arenas will be built, and new sports created/adapted for Mars (the moon will surely be popular for this too). There will be Martian olympics and various other games that nobody on Earth can do - but will surely pay $20 to 'livestream' ppv.
I'll keep thinking about possible _differentiated, diversified, and large-scale_ revenue sources for self-sufficient colonies on Mars to consider prior to and during their founding.