I think one advantage of the hyperloop is that it would probably be quieter (they claim the compressor uses <150 HP) so that would make putting it on a pylon simpler and would reduce the need for noise barriers. Also the separation between tubes could be much lower than the separation between tracks. I'm not sure how much this would affect the feasibility of putting a more normal HSR train on small, lightweight pylons, but it wouldn't help.
Also, it has a large enough speed advantage that it doesn't have to go straight to the city center to have a time advantage over air travel, and that would be a big part of reducing the cost while keeping it viable.
I mean his tunnel/tube is more expensive than a noise barrier. It has all the same properties but has to withstand far higher pressure differences and be built to insane tolerances.
