That's about 25 million households, or about $150 billion gross revenue (not profit) over a decade at the price assumptions you give.
If SpaceX’s public estimate of total Mars program cost of $10 billion was reasonable, that might plausibly fund the program. But it's highly dubious that that is even on the right order of magnitude.
Sure, but “rich people's vanity may help finance the program” (and NASA will help finance the program, etc.) doesn’t anything to support “satellite internet will finance the program by itself”.
I doubt it. There are a surprising number of satellite internet customers in completely illogical places, like in the middle of major US cities. A lot of this probably has to do with the abysmal state of broadband options in the US -- many customers are served by a single wired broadband provider; when they lose faith in that provider (or are denied service), satellite may be their next best option.
And there are people living rurally outside of the USA, too.
Starlink could bring rural Asia and Africa into the 21st century with one terminal per village.
A fifth of the US population lives outside of big cities. That's not a big fraction, but it's a small fraction of a very big number. Being able to capture that market even for a decade would basically bankroll SpaceX's Mars goal by itself. And that's not even looking at other countries.