After days of price research, I finally gave in and purchased a D-link DNS 323 2-bay home NAS appliance from Newegg, along with a couple 500GB Samsung drives, for a total of $415 or so (just over 80 cents per gig in a RAID 1 configuration).
Yeah that's a bit expensive compared to just sticking the hard drives in my desktop, but hopefully some of it will be made up by power savings. I was planning to use the desktop as a file server, since it has the most space, but now I can just run this tiny device, and set my desktop to go to sleep after a few hours.
As to whether the math actually works out. That's TBD. It also looks like a bunch of people are using Amazon S3 to do their backups, but I'm sure that's more expensive for hundreds of gigs of data, and a lot slower.
The cool thing about this D-link device is that it runs linux. And it looks like you can install stuff onto it w/o even flashing the firmware.