As with anything new, development costs can be brutal. It would take many hundreds of billions of dollars up front before there would be any revenue from asteroid mining and the like.
I dispute your costs. Nobody thought an individual could fund a way to Mars. Musk IS our space program now. As to funding we gave away that much money already and are only now figuring it out. It could be funded by several individuals and/or governments over time. It can be done. It should be done.
How much of your money would you give to save your life or that of your child? The Earth is at a point where we could end most if not all of the life here if we tried. Or a celestial event could wipe us out.
It is in our best interest to spread out into the solar system and universe. We have the knowledge to begin. We have the money. But do we have the courage any longer?
We once took up the challenge and explored and conquered our world. It is now time to conquer others....if we can find the courage and curiosity our forefathers did.