While falling short of the magical $100 student laptop, the newer computer mother boards are getting so inexpensive that you can build a variety of useful computers. The link below provides a list of equipment required to make a number of sub-200 dollar computers. Of course, if you want a full computer, you'll need a monitor, keyboard, and mouse. You'll also need an operating system. Given that the computer vendors, such as Dell, can buy Windows at a far lower price than can you, they'll be cheaper. But, if you want to build a powerful but cheap firewall, this platform can be an attractive solution. For a truly secure firewall, I would install a CD-ROM and run the OS and firewall application from it. No way a hacker is going to plant a spy program on a read-only CD.
Build a low-power x86 appliance for under $200!