It might be more valuable than spending it on the US military, which has over 35 times the budget of NASA.
Seriously, there's tons of more pressing issues back here on Earth, ranging from real, social one like inequity and discrimination, technical/political ones like the ever growing piles of garbage all over the place, healthcare, climate change and mass extinctions, and say, taking care of that whole cycle of oppression and bloodshed that has been going on for a while now (not that it'll go away, but we're pretty fucking screwed if we don't come up with something which will leave most people content).
None of those problems will go away if we send little men on tin cans to space, and in fact, I can think of very little problems that will be solved by that. We're a long way from escaping the mess which we have created here on Earth, and wherever we go we'll do it again, and that's being optimistic in saying that we will actually get a self-sustaining Homo Sapiens population off the planet before our shit catches up with us and we're actually forced to do nothing but deal with the consequences.
All that said - perhaps... perhaps... Perhaps the minerals and compounds needed to survive as a species are elsewhere in our solar system.
Not only that, but more than we could at this point imagine using in a long long time, and an inexhaustible supply of energy. I find it hard to believe that these resources wouldn't help us just a little with the seemingly intractable social problems. Those very often boil down to arguments over limited resources in the end.
In fact, I can't think of a major world conflict or problem that isn't at its root about the allocation of resources, and it's all there for us, right above our heads.