I have the full 20 volume set of the 1931 Grolier Canadian Book of Knowledge. It's in excellent condition: no spine separation, no page tears, no water damage. They've just been sitting on a shelf for decades, so home shelf wear applies, but that is just yellowed pages (and nothing of the print has faded). I've seen versions from the 50s, in worse condition, selling for over $500. How much should I be thinking?