From a physics standpoint ultra high altitude supersonic flight is not that much more expensive. The problems are mostly around economies of scale, but as soon as you get something that's competitive you will expect the price to fall quite a bit. A 400 passenger super sonic jet might use 2x the fuel per flight, but when it can make twice as many flights per day that's huge from capital standpoint.

That said, the concord was much closer to a prototype than what economical supersonic aircraft will look like. But, nobody is trying to build thousands of 400 passenger supersonic aircraft which makes your comparison more realistic.