Let's define "overpriced":
1. Too expensive to justify the cost despite quality
Some aren't willing to pay even more than 10$ for headphones - I don't think that's reasonable without caring about sound, craftsmenship...
2. Too expensive regarding it's performance
I agree, but "performance" is mostly subjective outside the labs. And the quality->price-ratio is non-linear, the performance difference between a 10$ and 100$ headphone is much bigger than improving the 100$-performance by spending 1000$.
3. Costs vs. selling price
Many products are manufactured with low-standards, using conventional design (low R&D), cheap materials and production technology with ridiculous standards (unskilled slave-labour). But instead focusing on expensive marketing and high margins for the brand.
These aspects also affect sound-quality and reliability and therefore are a good indication for "overpriced" products.
Sennheiser for example uses/used high-quality suppliers and craftsmenship for the HE90 (which was only build 305 times) and HD800. Spending a lot of effort into R&D (including expensive analysis of materials, simulation of all mechanical and electric components instead of simple trial & error) and having skilled and well-educated technicians in Wedemark (most of them are trained for over 3 years by Sennheiser and special "technical colleges" - not just "trained-on-the-job" like in Ireland or China).
The margins on HD595 or IE8 are most likely much higher - therefore I would call these products "overpriced". Not only regarding sound->price-ratio, which is a matter of personal preference but regarding the possibility to get similar technology/quality for a much lower price.