I think theoretically, you could make money refining rare minerals from asteroids (like platinum, gold, rare earths). The issue is that since the startup cost is so high, the mission is very risky (conservatively, it would probably cost in the 10s of billions to be able to refine anything at scale) and 10s of billions worth of rare minerals won't stay worth 10s of billions since the world only needs so much minerals - the price would fall substantially.

I think the only thing really worth doing in space, economically speaking, will be energy related. Maybe harvesting helium 3 will be lucrative. Maybe (I doubt it) there will be a profitable way to harness solar energy - could be more profitable if we can produce the panels in space by the asteroid they are procured from, but then the issue is transmission.

Actually, there is another thing. I think space tourism could be lucrative. Imagine if you set up a lunar colony for 20b that could house ~1k people with about 200 permanent staff. If 800 people are paying $100k/week (plus cost of transportation) to stay up there, you're making $4b per year. Personally if I were worth in the ~10s of millions I would absolutely shell out $100k to spend a week on the moon so I think this kind of thing could work