I agree with the notion of a PC. Heck, you could put together a headless linux box for $100 or less and put several drives in it . As a disk server it does not have to be powerfull, just make sure it has 100 Mbit lan connection of course. I've seen MB's with C3 1Ghz CPU, integrated lan, video for $5 before on
http://www.techbargains.com. This would make a great network storage box. Just add case,power,supply, and cdrom or usbflash disk or whatever to boot off of.