Have you seen the Goldmine Record Album Price Guide? It's not great for classical but is pretty complete for pop & jazz.
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Most published guides concentrate on recordings currently in print--meaning CDs only. But the allmusic website lists almost everything a given artist has ever put out, so there's plenty there that never made it onto digital. I've found a number of mistakes, but that's inevitable in a database of that size. Overall, a very useful reference.