There is no perfect sound and no perfect headphone. You can't quantify it.

The point where spending more money greatly diminishes returns is when you have zero headphone and then you have your cheapest headphone. Going from no headphone to $1 headphone is the biggest improvement possible and going from $1 headphone to $50 headphone is not nearly as great of a leap. That's why diminishing returns don't really make a whole lot of sense.

If you want to look at it like a graph where you plot audio quality against price, it's going to be a steep steep steep jump from $0 to the dirt cheapest headphones you can buy, and then still fairly steep but growing more gradual as you increase price. But again, this graph is imaginary because you cannot quantify sound quality.