IDK, indeed $X/10 is ambitious but he does have a decent track record with this already. SpaceX is already $X/3 or $X/4 and is on the path to $X/10 with reusable rockets, and it's also more ambitious (rocket reuse, highest payload in the world by 2x, new capsule has powered rocket landing on any surface in the solar system).
In my mind he's earned some benefit of the doubt. Of course, there's still a ton to prove, and the idea sounds looney. You'd also have said that about SpaceX 10 years ago, though.