It can be a tough one to call but it's whatever you're prepared to pay for them really. I sold a good set of grey with red/black stripes a couple of years back with door cards and I sold them for £500. They were snapped up after a week or so. But as with anything like these, good sets can command princely sums and there is many a joker out there selling shoddy seats with torn fabric for just stupid money. But these seem to attract the 'Scene Tax' status.
Take your time finding a set and be prepared to negotiate.