For what it's worth Musk believes that Starship will bring a >10x improvement for $/kg to LEO. Not throwing away your second stage will do magical things. At any rate a Martian colony would be mostly self-sustaining with trade being a relatively small fraction of gross domestic product (like with international trade right now). On planetary scales, moving trade from 0.01% Gross Martian Product to 0.1% would more than justify the development costs for such an engine.

I think we can safely say that we are very far from 1 Martian colonist, let alone 1e6. I think we would outperform expectations to set up a very small research station by the end of the century at great expense on the Martian surface. So there's clearly a lot of magical thinking going on here. If we allow for some amount of aspiration, there are a few ways to provide a return on long time scales (much more than 1 century, with capex far exceeding $175B just looking at the cost of staging the necessary propellant in LEO):

- Such a base will have a propellant depot in a much shallower gravity well than Earth and a far thinner atmosphere. This is a huge comparative advantage for launches into deep space.

- Necessity breeds ingenuity. A Mars colony would likely generate many advances useful to Earth that do not make sense to pursue terrestrially. Think hydroponics, insulation, radiation shielding, so on and so forth.

- There are plenty of completely desolate locations on the Martian surface - ideal locations for radio telescopes, neutrino observatories, and other hyper-sensitive experiments.

- ??? (We're trying to predict centuries ahead, after all!)

Interplanetary trade starts to make a lot more sense once a good chunk of your economy is happening outside the gravity wells of planets.

Presumably the people of the future will be much richer then we are. I think there's an idea that over time wealth doesn't grow in an absolute sense - it does. Shipping good overseas makes sense now in ways it didn't 100 years ago not just because shipping costs have fallen but also because incomes have gone up.

It wouldn’t make much economic sense to ship stuff back down a gravity well if you could make it locally.

On the other hand it might actually become cheaper to ship stuff to Earth orbit (even LEO) from Mars, simply because of the delta-v cost.