Anyone (everyone) care to throw a dart?

...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!

@arizonabob I too have pondered this very thing from time to time. I feel like you get to a certain price point (not entirely sure what the number is though I guess it's different for everyone depending on what you can afford) where to get the slightest bit more detail (for lack of a better word) from your system, you literally have to pay double what you have already spent. The question then becomes are you willing to go there. Using speakers for example, I currently have Dynaudio Evoke 50's. To get to the next level I'd have to move up to Contour 60i which are literally double the price of Evoke 50's. How much better can it get, double thereby equaling the cost? For me no way. Not possible. Are the 60's better, have more detail, soundstage, etc? I am quite sure even though I have never heard them, but I can't afford them so I don't even think about going there. Regardless I'm happy as a clam with my Evoke 50's. But I totally get your post as I too have had that same thoughts...

If pragmatism is your main concern and you can afford the expense, I think your concern is irrelevant and, ultimately, non sensical. It seems to me that as we age and our hearing gets worse, that this is precisely the time to compensate for our hearing’s diminished resolving ability by investing in equipment that is capable of MORE resolution and refinement. 50% of 100 is more than 50% of 80. No?