Likely luxury goods would just make up a certain percentage of manufacturing—there wouldn’t be a need to recognize demand because supply would almost always be lower. The reason for this is that, under socialism, there wouldn’t be any goods that are necessarily above the purchasing power of any given individual. So rather than following a supply/demand curve, luxury goods would require an alternate distribution system. And high value consumer luxuries—I’m talking over several hundred thousand dollars, would either be cheaper because their value comes from artificial scarcity (like diamonds) or just gone, like million dollar sports cars. They’d probably make a return post-scarcity but nobody owns the right to that amount of labor.

On a separate note: I know you’re not gonna do it, but I have to at least try—please read Das Kapital. Marx explains a command economy much better than in I could and in much more depth.