High end audio amps in general cost a lot more than what you see in guitar or PA amps. The main reason is that they are designed to perform at much lower levels of distortion, which necessitates things like big custom transformers, electric parts like resistors and capacitors that have much more narrow tolerances, a lot of times special and costly wiring. All this costs money. Then you add in the engineering time to design circuits that try to minimize the amount of feedback loops used, more expensive chassis to try and eliminate vibrations, and that they often are handmade. And, because they are trying to drive speakers cleanly over a wider spectral band, they are usually of much higher wattage, so that you don't cross over to break up until truly ear splitting SPLs.
Guitar amp generally have a very different design goal: to distort in an interesting way. Doing that generally means lower cost components, and a much more forgiving set of design specs.
The SS McIntosh amps were also voiced to sound more like tube amps than most. There are a few more SS audio amps out there voiced similarly, some with mosfet circuitry, that are cheaper. But I really wonder if those would deliver in the same way.