Anyone (everyone) care to throw a dart?

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.

I only use my hearing aids at social gatherings/dinner in a good restaurant/etc.   I don't listen to music with them..they close in the sound, and much too digital sounding.

...anyone care to throw a dart at the price point where diminishing return kicks in?

for what?

Price is not the arbiter of good sound. Smart buys and common sense help. Listening to and tuning your rig are critical factors in getting the best from your source. The most important thing is a great recording. I guarantee you a 1967 Audiophile Records Doc Evans record on my system will out-perform an average  digital track on a million dollar setup all day every day. 

$100...for my wife. 
I way way past diminished, and not quite the OP's age... 
Whatever is fun!