Asklemmy

"What is the return on investment?"

In the case of 3Body,

spoiler

it wasn't about profit but the about the survival of the entire ecology. The planet was doomed to fall into one of the stars and so the race was picking up and moving as much as it could to the next-most-habitable region.

That's only very loosely a "profitable" enterprise. Certainly, the initial generations won't see any kind of profit simply due to the length of the journey.

The same principle applies to other planets – if it's profitable, it will be pursued.

But a practical ROI can only really be measured within a single lifetime. And extraplanetary travel will always have a return of $0, as anyone deciding to perform extra-planetary exploration today will not see the benefits for generations. One might argue a more Ursula LeGuin-esque view of interstellar colonization - as a struggle for survival that simply expands beyond the frontiers of a single planet. But then, what we're really talking about with Martian colonization or extra-Solar travel is some kind of politically or ecologically motivated Exodus. Because the economic exploitation of the New World was mostly just hit-and-run raids early on. The Virginia Company was an abject failure as an economic exercise. It cost far more to maintain than it yielded.

The real motivating force behind early colonization was the 30 Years War and the flight of the Protestants. What you're ultimately going to need are some Huguenots with space ships. Even then, the real labor force in colonization were indentured servants and slaves. And there's not going to be a Trans-Atlantic Triangle to move people from Earth to the spacial frontier, because... Its space. There's nothing out there.