Nice write-up about how you can re-use old routers (assuming they are DD-WRT compatible) as a networked controller. I have done similar stuff in the past. Its not an RPi, but the routers sometimes can be had for $5 at yardsales, and low as $15 on internet sales.
http://www.instructables.com/id/Repurpose-an-old-wifi-router-for-the-internet-of-t/