Is 200$ top graded speaker better than a 400$ 4stars/gradeB/call-it-what-you-want/next to the best in the line speaker?
You've hit on a tough question, and one with which many, (most), of us have dealt.
1. Yes, there is absolutely a "point of diminishing return" on speakers, and all other stuff. At the low end of the prices you can get a big improvement for a little more money. But as the prices get higher, it takes more and more money for a small improvement.
2. Sound is subjective. Everyone hears it differently, has their own preference and it cannot be measured. It is because of tone or timbre. Think about it this way... A piano, violin, guitar and trumpet can all play the same note, yet they sound quite different. Speakers are like that. Equipment can measure how accurately they play the note, but not which one sounds better.
So yes, the best answer is always to listen to all speakers and decide which ones you like the best. But that's not possible. Even listening to just the top recommended 2 or 3 can be quite difficult. So what to do?
This was my approach. I'm a common audio guy. I don't tune pianos for a living. I don't play first chair violin in an orchestra. I listen to all, (most), kinds of music. So I started reading forums about audio & speakers. I found patterns. There are speakers
most people like, and speakers
most people dislike.
Among the speakers most people like, it's easy to find people who prefer any one over any other one. And it's easy to find arguments over which is better. But even in those arguments, people still agree both are good.
I picked my budget and narrowed to speakers within that budget, and I looked for one particular thing. Could I find more than one opinion that a speaker was "bad"? Meaning, even if some people preferred a different speaker, did those people think my prospect was a lousy speaker?
That's my suggestion to you. Pick a budget. Read reviews AND common forum members opinions. Find speakers in your budget that nobody says are bad, even if some may prefer something else. Then, if you can actually listen to one or more within that list, do it. If not, consider shipping cost, warranty, return policy and customer service reputation. Pick one, and enjoy.
One last note. If your example is accurate and indeed $200 is your ballpark budget, you're very fortunate. There is one speaker for $200/pair that I think everyone agrees is the top choice. You can see it here:
http://philharmonicaudio.com/aa.html