"I'm a believer"

There is no rational at all for me to put a price tag as you did for speakers diminishing returns threshold ...

It is a subjective threshold linked to each person audio history and needs ...

It is an objective threshold also related to the way each speakers is mechanically, electronically and acoustically embedded in a room ...

I will not speak about the huge transformation of any speakers when it is put under acoustic control; i can for example speak only about the vibration/resonance problem put under control ...

Doing so with vibrations control with any speakers will change timbre and imaging and soundstage and well done will transform most speakers completely ...Add to this the electrical noise/signal ratio control and the room acoustic and any relatively good speakers at relatively low price will rival or trail not so afar any costlier speakers especially those not well embedded as it is generally the case ...

Give me a straw and if i put it at the right location i will change a room or a speakers performance ... I did it ...Mechanical equalization exist too ...I use it right now ...Electronic equalization dont replace mechanical equalization nor the reverse ...

The pitch discourse of speakers sellers dont include the mechanical electrical and acoustical NECESSARY embeddings for ANY speakers to do his optimum S.Q. Consumers dont like to be informed that they must WORK, EXPERIMENT AND STUDY after buying ... 😊

Some speakers do better than others without any embeddings controls, that does not means that they dont need it ... And speakers well embedded reach way more  easily the minimal acoustical satisfaction threshold ... It is why the diminishing returns point is way lower than most people think about ...

There is a point of diminishing returns with loudspeakers and I put it in the 40-50 thousand dollar range.

Yeah, unfortunately, price and quality do not always correlate, but real quality is always expensive, and there is no reason why it shouldn't be. Question is how expensive is it to you ? Class war is coming.. 

Quality of design does not mean ACTUAL quality of sound ... It means ONLY potential quality of sound ...

Actual experienced sound qualities need controls over the three working dimensions of any gear at any price to be OPTIMAL : mechanical,electrical and acoustical ...

Quality of design is not in any way LINEARLY correlated with price tags ... This does not means that there is no relation between price and sound quality ...

Put all this together and the question about "how expansive is it to anyone" to reach some satisfaction level had not so much meaning in term of price...

All vintage speakers at low cost are not trash ...As my Vintage Tannoy dual concentric Gold , a design marvel ...

All contemporary low cost speakers are not all trash either ... My actual low cost modified and well embed small speakers beat all headphones i ever listened to and their basic price 12 years ago was 100 bucks ...In spite of their good reviews i disliked them a decade BEFORE modifying them because i had no other speakers to work with ...

Their ratio sound quality/cost is insane AFTER modifications of the resonator box with a new refined and more complex porthole and after mechanical and electrical and acoustical embeddings ...

Now very costlier speakers will beat them for sure as my Tannoy would have too well embedded but not by a so far margin ...

it is why i am happy with them ...

You can laugh , it is me who laugh the last ... 100 bucks speakers with 4 inches woofer in the race ( 50 hertz now ) ...

There is a minimal acoustic satisfaction threshold which is correlated with the diminishing returns  zone ...

Diminishing returns can suddenly materialize and bite you in the ass - just look at EV's now. Same with audio equipment which is always subject to unforeseen technological displacement. You hear lots of cheerleaders now touting the demise of class A and AB amps in the face of GaNFET's. They may suddenly gain enough momentum to cause a feeling of diminishing returns for everybody in possession of outmoded technology or what is perceived to be outmoded.