Strictly compared to what is out there, I don't believe they are overpriced. I don't believe their performance is $1200 greater than the your average $300 headphone, but you are probably getting one of the best neutral and detailed TOTL headphones in production which you most likely will not need to upgrade. Also, one of the most popular, so is fairly easy to resell for 80-85 of the purchase price.
Stax wins the overpriced vote from me, though no doubt it is quality. But for HD800, you do not have to buy an electrostatic amp (which would only work for electrostatic headphones), resulting in a $10000+ rig. You do have to find something that works with HD800, but it will not be anywhere near $10000 (okay, it MIGHT be). Plus, whichever headphones you already have, said amp might also be used for that as well.
I am running mine out of a vintage amp that ran me $300 and it is very sexy. Money well spent, although I still frequently go to my Audio Technica AD2000x and IEM's as well.