Best Buy had a nice deal for a 300GB WD drive for $70 after rebates. Unfortunately it's out of stock pretty much everywhere. You might get lucky, though.
As for the size difference in what the drive claims to be and what it actually is, storage is generally measured in multiples of 1024 (which is a power of 2), but manufacturers will commonly use the "technical" measurements, which are multiples of 1000, so, for 500 gigs, you'd have:
500 (gigs) * 1024 (megs) * 1024 (KB) * 1024 (bytes) = 536870912000 bytes
vs.
500 * 1000 * 1000 * 1000 = 500000000000 bytes.
You'd need about 540 "gigabytes" to actually get 500 gigs of storage.