For a simple one source system, either analog or digital, speaking new equipment and average medium size US room, I would say $100k is enough to get a great sound. Spending more may not be worth it.
Anyone (everyone) care to throw a dart?
I understand my hearing at 69 isn't even close to what it was at 19. I'm sure your results may vary. However, anyone care to throw a dart at the price point where diminishing return kicks in? There must be one out there. Regards, AB