Building a working R2 of your own will cost about $500 to $2,000 and at least a couple months worth of labor on the weekends. Fortunately, there's a much easier alternative to getting your own astromech. Habro sells the R2-D2 Interactive Astromech Droid, a 10.5-inch version of R2 that is has voice-recognition, location sensors, and a navigational system powered by sonar technology.
This R2 unit was actually launched back in 2002 but it sold out quickly and Hasbro had supply problems, so it was very difficult to get a hold of one of these droids back then, unless you wanted to scour eBay and pay over $200. It was re-launched in September 2006 and has not suffered from the same supply problems, although there are times when these are difficult to find. There aren't sold everywhere. The most popular place to get them is at The Sharper Image, which is where TechRepublic found one.