OTOH, there's a lot of stuff that is far more feasibly moved on the Earth's surface rather than in flight. So some sort of surface-level infrastructure will be required: highways, railroads, canals, rivers, oceans.

Some of those are amenable to high-speed variants, some less so. And once you've established transport corridors, much of the hard part (rights-of-way acquisition) is accomplished, so that adding a high-speed route becomes far more tractable.

For the most part, actual construction costs are fairly modest, modulo bridges and tunnels, both of which can of course be quite expensive.