What kind of HDDs are we talking? Are you looking for enterprise / pro NAS drives? New only or is refurb ok? I know goharddrive has great discounts on enterprise refurbs, and often gives their own warranty, even if the manufacturer warranty expired. I am no sure if there is a volume discount exactly, but there might be. I ordered 4 12TB WD data center refurb drives from them a while back for a NAS system.