Found a bookplate-signed edition of this book on-line. Ive seen this seller around and his stuff seems to be out of sight generally. If this book were actually signed this price would be really low, but because its bookplate signed, I think hes asking a bit much. Im not sure when Easton started printing either edition, but Im farily certain Herbert was dead (1986) by the time they did start. Ive seen one with McNelly's signature (McNelly was Hebert's archivist, and was an SF scholar and anthologist) that went for about $150 or so before.
signed by 8, Easton Press Dune, signed by Frank Herbert - eBay (item 190260027066 end time Jan-14-09 18:30:39 PST)