Hypothetically, if the US were to put 20% of their budget into (truly) getting people on Mars, they'll do it.
If they put 20% of their budget into eradicating poverty?
Right now the US government puts between 10% to 30% of their budget into Medicare and Medicaid.
Is poverty eradicated?
Which is not how it works.
The cost of the actual raw materials to build almost anything is a tiny tiny tiny 1% fraction of the cost of what the thing actually costs to build. Rocket fuel, titanium etc. are all cheap compared to the cost of actual rockets. So the question of course is, where does all that extra money though? And the answer of course is: into people, process and technology.
The cost of rocketry, or particle accelerators or fundamental science, is correctly viewed as a massive sponsorship and investment into people and tools to improve how they do something so they can do it better.
And that is why "spin-off technology" actually happens in the first place - because large projects mean that we have now skilled up, tooled up and improved the capability of vast numbers of people, companies and fields of trade - which is what makes new innovations possible, or changes the market dynamics so things become economical and then profitable.
As I thought about how he is perfecting rocket technology, I wandered off on a tangential rocket technology (thought) that would tract nuclear rockets launched from the earth's surface and curry them off into space... and return home.Is it a good idea? It is if it works. Do I plan on working on it? I don't know shit about rockets, so probably not. The point it, we need more "Mars Men" in order to discover the ideas that will change things here on earth. Maybe Musk won't cure cancer or end wars but maybe someone inspired by him will.