Finally built a spreadsheet to calculate the true disk foot-print of some subsets of 6 pieces, now the real disk space with dependencies are correct. See the web site.
Can't quote for a 1.5 TB disk with tablebases as my distributor does not have pricing from Seagate just yet but should be available soon. I expect WD and HitachiGST to respond with 1.5 TB disks soon.