How much do you have to spend?

That’s where I ended up—satisfied that returns would be diminished with rising cost. $16k. But my rigs are somewhat esoteric. The primary one breaks down a bit like this:

Turntable $3500, tonearm $2000, cartridge $1500, phono amp $2000, preamp $2000, amplifier $2000, speakers $3000. + cables etc.

Obviously, you can spend a lot more and still run into diminished returns. It's all relative. However, in my case, I've chosen and upgraded many well-regarded vintage components. 

One could argue that in high-end audio diminishing returns start the moment one goes above box store (average consumer) offerings.  

for about 200K build a room that floats in a pool of jelly to avoid any vibration from the Earth. Otherwise it makes no sense to buy any audio gear.

@fritzenheimer 

You're asking people who would never have that much money to spend in the first place!

A perfectly satisfying HiFi system can be assembled for $5K or less by hunting for gear on the secondhand market. I stopped buying new a long time ago.