I don't think we need a profit motive. Egyption pyramids would never pay off, and im willing to bet a lot of their construction was just for the cool factor and "look at what we can do collectively because we have so much excess of time and goods!"

History is full of monuments and art who's only purpose was to be cool. And sending humans to another solar system would be a pretty cool monument to humanity.

It is more of a PR problem really to convince people to contribute.