For a rough guideline at least, try Mac2Sell - Argus, Evaluation, Guide to used Mac, iPad, iPad mini, iPhone & iPod - Occasion
mac2sell overprices mac pro's anyways, so their quotes are useless. worse than useless because they may give a false sense of value to the seller.
i sold an 8 core 2.26 a few months ago for $1200. yours would be higher, but you can use that as a baseline.
mac2sell quoted $2500 for the 8 core 2.26