Since hard drive prices are still insanely high after the Thai floods, I have resorted to buying cheap external drives. I simply remove the drives from the enclosures and use them bare.
These drives cost around $110-120 which are cheaper than their internal counterparts. I'm still flabergasted that external drives cost less than internal drives. Seagate now supposedly uses the 3TB ST3000DM001 7200 rpm drives in these enclosures.
The only downside is that you lose your warranty. With one year warranties on most drives, I am willing to take those risk for my personal archival needs. These will be great in a Drobo 5D I plan to get.