3TB Drive for $140 (< $50 per TB)

http://www.buy.com/prod/western-digital-my-book-essential-3tb-usb-3-0-external-desktop-hard/q/loc/101/217438518.html

The sweet spot seems to be moving to the 3TB drives.  With free shipping this is at $47 per TB.  Also this would increase our current system from  8TB per plugbox to 12TB per plugbox.