You should read this:
http://www.headfonia.com/sennheiser-hd-updated-drivers/
http://www.headfonia.com/the-sennheiser-trio-hd580-hd600-hd650/
http://www.headfonia.com/sennheiser-hd580-affordable-reference/
As for prices and threads like these influencing them ... I don't think it's the case.
I have bought a pair of HD580 for 120 euro (shipped and all), which, in my book, is very cheap .
However they were the old version, had a few signs of wear and needed pad replacement.
The second pair I've bought was in mint condition, had the new drivers and sounded quite a bit better than the old (and identical to HD600). But these cost me over 160 euro.
Overall I must say that HD580 is a bargain of a headphone, but as always, you get what you pay for.