Affordable vs. ultra expensive speakers - what's the difference?

There are certainly differences, in every aspect of sound quality. 

But if one is expecting the differences to have some sort of 1:1 (for every $ more I spend, I will get exactly that much more improvement) correlation, that will not happen. 

The higher up the scale one gets, the less the improvements will get. A $50K speaker, will not sound twice as good as a $25K speaker. But, it will probably sound quite  bit better than the $25K speaker, than a $100K speaker will over a $50K speaker.

That being said, the difference the YG Hailey will have over the Monitor Audio, will be appreciable. I have heard the YG's plenty of times, and I have heard Monitor Audio before (not the ones in the link), so, I am speaking from pretty good experience. But, pretty much all aspects of preproduction will be better with the YG.

Yeah, with regards to audio, I really don’t believe there is an objective point of diminishing returns.

It is up to each individual to decide where that point is for each of us.

My rich cousin recently upgraded his previous speakers, Magico M3 ($80K), to the a pair of Von Schweikert Ultra 9 at $200,000.

They sounded slightly better in almost every aspect, and all those slight differences added up to a more compelling, involving listening experience. Was it worth the more than double the price for those small improvements? It was for my cousin.

And let me add, that the vast majority of music listeners out there, listen to MP3s on $20 dollar ear buds, with their smart phones, who think that those here who have only spent a few grand on their modest systems, have surpassed the point of diminishing returns.

@cakyol 

The difference = a lot of money for about 2-5% improvement in sound.....

The curve of diminishing returns.

As I said earlier, while this curve does exist, it is highly subjective, and is different, sometimes drastically so, for everyone. 

For some people, even people that can't afford the difference in money, that 2-5% difference you posit, may actually stick out like a sore thumb.

Some aspect of audio reproduction, that you may find inconsequential, for others, may be one of the things they place at very high importance. And of course. vis versa.

Remember, the vast majority of music listeners use $20 ear buds, to listen to MP3's, on their smart phones. Or they listen to all their music on their Alexa, or similar while at home.

I am sure a substantial percentage of them think that those of us here, who have modest systems, no where near the very high end, are at the extreme end of that curve of diminishing returns you mentioned.