You’re right about the cost, but you’re forgetting two factors:
1 — Earth will end someday. That’s not an if, but a when. It may be a long time yet, perhaps a billion years or more…but more likely much sooner, given the frequency of supervolcano eruptions (like the Siberian Traps eruption that killed 95% of all life on Earth), the ever-present possibility of an asteroid or comet strike (check out what Comet Shoemaker-Levy did to Jupiter 30-odd years ago — such a strike would have ended life on Earth), and then there’s humanity’s own greed leading to the certain destruction of our own habitat.
And if humanity remains limited to Earth, then when Earth can no longer support life, humanity is done, ended…and every work of art we’ve ever made, every dream we’ve ever had, will be not just ended, but erased. Permanently.
2 — Humanity spreads. Humanity grows. It’s what we do. Sure, you can call that the definition of a cancer if you like, but when you look in the eyes of not just your children, but especially your grandchildren, you feel the happiness and the joy…And you begin to understand the why. Humanity will grow.
We can reach for the stars and continue to propagate for the foreseeable eons…or we can remain on Earth and consign ourselves (and our children and grandchildren) to an eventual doom more certain than that faced by any death row inmate in human history.
There’s absolutely nothing wrong with efforts to make our civilization as green and sustainable as humanly possible…but it is no exaggeration at all to say that forsaking our species’ spread to the stars just because it’s so much cheaper to make Earth more livable is quite literally ensuring the eventual suicide of humanity as a species.
Sure, we can tell ourselves that such is the way of things, all things end, and better to forsake the stars if it means an extra loaf of bread for the hungry, to stave off the kind of hunger few Americans have ever felt…
…but as much as I’d love to do that, if we must choose between feeding the poor and reaching for the stars, I choose the latter. If humanity is a cancer, then I choose to metastasize.
BUT the choice you present is a false choice. We don’t have to choose one or the other. We can have both. The amount of money we spend on space is far less than what we spend on armaments. Bring our insane levels of “defense” spending back to some semblance of sanity, and we’ll have enough to feed the poor and reach for the stars.