Western digital has their 4TB WD Red CMR drives with 128MB cache for $68.99, although currently backordered, it might be a good alternative to the WD Purple drives. Generally, the 2TB WD Purple drives are in this price range ($65 - $80). I've had good experience with the WD Red drives as well.
WD Red Plus 4TB 128MB CMR HDD
www.westerndigital.com
WD Red Plus 4TB 128MB CMR HDD

WD Red™ Plus NAS Internal Hard Drive 3.5" HDD | Western Digital
Packed with power to handle the small- to medium-sized business NAS environments and increased workloads1 for SOHO customers, WD Red™ Plus is ideal for archiving and sharing, as well as RAID array rebuilding on systems using ZFS and other file systems. Built and tested for up to 8-bay NAS...