I'd lock that up; they're rare. Even incomplete, there's a big gap between $600 and a few thou. Though I bet you can get a hundred or two off; seems if it's priced at $600 the guy doesn't know what they do go for. Last-generation spinner racks (y'know, with those white plates with heads of Archie, Superman, Spidey and Richie Rich on the top) are tough enough to find and go around $500 last time I had reason to check. Might be what he's basing his price on.
Maybe try to point out some nameplates are missing and try to get a price drop, but if I could not get him to budge I'd pay the $600 and take it. But that's me. But I think price-wise you're doing well and it's probably scarcer than most of the books it ever held.