Sets of almost anything, from encyclopedias to collections of novels, have really dropped in value in the used book market- and used books have not been doing that well, anyway. Though if you search Abe Books, you'll find some pristine sets with significant prices I think it's quite possible that if you were to check some individual book stores there might be some older sets in poorer condition. Condition is a very important aspect of book value, and likely , given the popularity when they were printed, , poor condition sets of The Story of Civilization with a low price are still to be found.

But though the last time I browsed them I found the volumes on the Reformation still pretty interesting reading, of course these are pretty dated, and don't reflect current scholarship.