Thanks very much for the info. Bigelow Aerospace and Stratolaunch Systems are excellent examples. Bigelow is especially interesting because from what I've googled he doesn't seem to have any sort of technical background.
On the other hand, I don't believe John Carmack is a billionaire, and in any case Armadillo Aerospace has only had a few million invested in it. (Whether or not these guys are billionaires is very important. The question is about rarity, and the pool of people under discussion gets much bigger if we consider people with merely 100 million and who are consequently restricted to more modest goals.)
In any case, I think the point may still stand that these people devote much less of their emotional and temporal capital than Musk. I'd love to hear alternative evidence.
EDIT (in reply to jlgreco since I can't do it in the normal way): Yep, according to the side bar on Wikipedia his net worth is 2.3 billion as of March.