As of the writing of this post, the dollar/gigabyte ratio should be about 0.20 . That means if you're paying 20 cents or less per gigabyte, you're getting a good deal. Why 0.20? This is the average ratio for hard drive deals that I consider to be "hot". So, if a 500 gigabyte drive is selling for $100, it's a good deal, because $100/500 = $0.20/gigabyte. If you found a 400 gigabyte drive for $75, the ratio is $75/400 = $0.1875/gigabyte. That's an even better deal!
This ratio is for 3.5" Western Digital, Seagate, or Maxtor non-OEM drives; SATA II or PATA/133; 16+ MB cache; 7200 rpm; external or internal.
Of course there are other things to think about. You should be looking for drives made by Western Digital, Seagate, or Maxtor; they all make fine drives. And whether it's OEM or Retail usually makes no difference on the quality of the drive, although you might not get cables and such with the OEM version. You need to make sure it's the size/write format for your computer. If it's a laptop, you'll need a 2.5" drive, but are more expensive and are not covered by the above ratio. That is only for 3.5" drives. You also need to figure out if you need SATA or PATA. If it's SATA, don't get anything less than SATA II; if it's PATA, don't get anything less than ATA/133, unless your IDE controller is slower. Then, ATA/100 are OK. You should make sure the drive has at least 16 MB cache and is rated at 7200+ rpm. The days of 8 MB cache/5400 rpm are over.
Some people HAVE to have 5 year warranties; I think this is ridiculous. Why? The data on the drive is what is the most important. If my drive crashes after 3 years, it will be the hassle of buying a new drive, installing it, putting the backup data on it, etc. My time is far more valuable than the cost of the drive (and consider that it's now 3 years down the road; just imagine how cheap they are). And even if the manufacturer does replace the drive, they'll probably send you a refurb. You do not want refurb drives. So for me, if the price is good enough, I'll take a 1 year warranty, otherwise, 3 is fine.
Watch the forums. You might just find a deal less than the above ratio!