Law Of Diminishing Returns?

I'm curious about what you enthusiasts think of the product or price that eclipses your definition of "value".  

As an example I have a rich buddy that just spent 100K upgrading his (former) Pass 600s / Bryston / B&W Signature 800s / JL Fathom 8 speaker  system. I have a discerning ear and cannot hear the difference between the old system and his new S5M Perlistons (4) , Anthem AVN90, ,ATI amp AT6005 (4) and four subs.

This got me to thinking- 80% more money for maybe 20% more sound quality? 

Where is the sweet spot for the discerning ear and the affluent but not Billionaire (think Doctor/Lawyer/Indian Chief) budget?  Can you get 80% HiFi sound for 20K or do you need to spend 100K to get that HiFi sound?

-Asking for a friend :)