Ever since the floods of Thailand, I've been hoarding drives as they go on sale. Just for me, I probably have over 60TB of drives in various sizes. These 4TB Backup Plus drives were dirt cheap the other day on Amazon for $140. 7200 rpm, SATA III 6Gbp/s and Thunderbolt upgrade-able.
These external drives usually cost much,much less than their internal counterparts. There are plenty of youtube videos on how to crack open these enclosures for you to use as internal drives.
I may just want to upgrade my Drobo 5D and replace all the 3TB drives.