Diminishing returns can kick in rather quickly, but it really depends on your specific needs, expectations, and listening environment. And then how do you quantify it? What makes a given speaker twice as good as another? 50% better? 10% better? This is virtually impossible to answer in any objective fashion. Even the Harman "preference score", which was formulated out of decades of exhaustive research, is flawed to the point that its use is cautioned against by the people who created it.
I'd say the more practical question to ask is what your requirements are, and then you can go forth and research the cost of speakers that meet them. This could result in $200 or $2000 or $5000 or $10,000 speakers, etc. And then consider the possibility that a specific $2000 speaker may sound virtually identical to a $5000 speaker when both are played at moderate volumes, but the $5000 speaker has the ability to get twice as loud. If you don't need the additional volume, you've already hit a hard diminished return after $2000. But if you do require its more robust SPL abilities, the $5000 speaker may very well be worth the expense.
In other words, there's no real answer to this question. It always depends.
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