On one hand this is easy, as one can determine that Freddie Mercury or Billie Eilish is singing on any old PC or phone. Cheap devices are 99% accurate as DACs and amps. “Free” products are pretty decent these days…so technically diminishing returns certainly start by $100. With hard-to-drive headphones you might need to spend up to $200 to $500 for “good” quality.
I’ll defer to those with greater experience than myself with the ultra-expensive-esoteric stuff (e.g., @ProfFalkin above). The thread from @Torq below includes several of his lists on ‘best’ headphones, amps, DACs, etc. He has much more critical experience with weird pricey stuff than most people.
Also see this video from @Currawong on Youtube. He makes similar points about the THX AAA 789 versus much more expensive products.