My problem with this framing of the argument is that it's reasoned from the position of "If we want to do X then how much do we have to spend to make it happen"

My problem with this framing of the argument is that it's reasoned from the position of "If we want to do X then how much do we have to spend to make it happen". It's very much a "moonshot" style of thinking where you are only considering doing it once, it's not sustainable and it's not scalable. What we need to be asking is "If we want to do X what systems do we need to put in place so that X is trivial". Then the goal becomes to build sustainable and profitable industries that make everything leading up to X easier, cheaper, safer, and more accessible to everyone. It's not as direct of a route but it has a compounding effect built into it that works for you and eventually does all the work to keep X going. This is exactly what SpaceX is doing with the eventual goal of putting people on Mars. They've been finding and solving problems the whole time but the ultimate problem is that we still have all of our eggs in one basket and we're one asteroid, volcano, war, or pandemic away from civilizational collapse. Creating a self-sustaining population off planet is the only way to solve that problem.