You go to ebay, you do a search for Sigma dp1 quattro, you look on the left side for a checkbox named "Sold Listings", you check it, you get the average of the prices and you have your answer.
For us US buyers - no, it's too expensive. It's usually $650-750 new depending on the time of the year, Sigma's mood and the different rebates. It was at $599 several years ago (when it was freshly released) on a Black Friday. That night B&H made a typing mistake and actually sold it (and honored the sales) at $499 for a few hours.
So for a used one, I'd look at a range of $500-600 (here, in the US).
Cheers