That's a pretty high price for them lately: in general I see the complete sets of MoZ1-4 go for $25-45-ish, though it really does depend on the bidders.
If you're an
RJK fan, you may be interested in the
d20 versions of his books, as well: MoZ1-3 were published by Necromancer Games, Garden of the PlantMaster by Kenzer & Co., MoZ4 by Tadashi and I for Different Worlds (see
http://www.diffworlds.com if you'd like a signed/numbered copy).