That price for the Sennheiser is pure fantasy eh? I mean I can sort of understand why Stay & Adeze & Hifiman have to charge high but there's no way Sennheiser need to charge as much as they do for the HD650's. I would only buy that particular can second-hand. They aren't even built that we'll from what I've read. They do, apparently, come with a free veil though.
STAX and Audeze have special drivers to manufacture, so it's not too surprising to see the price increase there. Building your own electrstatic headphone is no easy feat and any dust that gets on the micrometer-thin diaphragm can screw up the sound.