Prices on large hard drives have improved slightly. Even 6TB drives are now at reasonable levels (given how new they are). Prices have remained stable on 4TB drives, but the 5TB and 6TB are now dipping slightly lower.
Prices at OWC (which I prefer due to their 90-day DOA replacement policy):
Many users will find 4TB drives ample, particularly in a unit like the OWC Thunderbay (4 X TB). Yet the 5TB drives are about 25% faster and the same price per gigabyte; this is 5TB drives are what MPG uses today.
For data intensive uses, buying too small for needs means replacement before the service life is over, so it’s best to assess current needs and rate of growth when choosing capacity.
MPG likes to match backup drive capacity to a matching-size volume (which implies partitioning large storage devices into volumes no larger than the backup drive capacity). And for RAID users, MPG strongly recommends a cold spare.
For a high capacity portable bus powered mini drive for travel or for desktops, MPG likes the 2TB OWC Mercury Elite Pro Mini.