Prices on large hard drives are getting interesting. Even 6TB drives are now at reasonable levels (given how new they are).
Prices at OWC (which I prefer due to their 90-day DOA replacement policy):
The larger capacities come at a premium for $/GB, but this has always been true. Buying too small for needs just means a proliferation of too many drives, so it’s best to assess current needs and rate of growth when choosing capacity.
For a high capacity portable mini bus powered drive, MPG likes the 2TB OWC Mercury Elite Pro Mini.