First off, I didn't say anything about someone living on Mars, as in, moving there permanently. That may or may not happen -- in the long run, probable. I just bet that SpaceX is probably the lead contender for who's going to put the first human footprint on Mars. It will likely be their tech, anyway.
Secondly, I'm pretty sure there are people who would love to live on Mars. At least give it a shot. Yes, in purely practical terms, it has a lot of downsides. But so does Antarctica, and many people have travelled there and some people actually live there at least some duration.
Third, there's good reason to believe, based on past experience with similar efforts, that the net economic effect for folks back on Earth is going to be positive, due to the side effects and fallout from R&D breakthroughs and engineering optimizations. That's what happened with the US-Soviet space race and the Apollo program. Also, since we're talking about SpaceX, which is a private company, it's possible that some, though almost certainly not all, of their funding comes from non-government sources, including non-US sources. It will probably be a mix. Mr. Musk personally put in a lot of his own money, which he made privately, to bootstrap it.
Fifth, one argument for establishing a permanent human presence on Mars is so that humanity has at least one outpost off Earth. So that if some disaster happens with Earth, not all of our eggs were in one basket. Paying even $100m to buy that seems like a pretty cheap form of insurance. Much much greater sums get spent on say NFL football merchandise, or pop music albums, each year. And certainly a couple orders of magnitude more have been spent on US military operations in the last decade, most of which could be argued were unnecessary.
Ninthly, because I cannot count: there are likely many economic benefits to you, the US and the planet to both setting foot on Mars and creating a sustained outpost there, in terms of the follow-on effects of inspiration and imagination and ambition, especially in children and upcoming generations. Surely if we could tip even 1m more children world-wide over into eventually becoming engineers, scientists and inventors, rather than lawyers, accountants, brokers or rap stars, the whole of humanity would be better off, on the net.
Eleventhly, I have no eleventhly. :)