Well, it's not like an extra couple ports of whatever type they use should drive the price up that much. Regarding the HTTP, FTP etc, that's all software so it doesn't increase their production costs any. The only real added cost I could figure is the bigger case and a couple extra power and data cables. :-P
I just went and bought 4 250GB drives and I'm gonna put my own together. Which Tom's article are you referring to? I was going to document mine and turn it into a howto, but maybe Tom's already has a good one?