from past experience I think it depends on what kind of budget your on, right now I think Admin is finishing up his $50 robot tutorial, and I'll bet unless you have any parts lying around from previous projects, that about as low a cost you can expect to have for a beginner microcontroller based robot. Also just as a question, what if any programing experience do you have? I have taken a couple classes of C++ so I'm not even close to being an expert but if you plan on making some kind of "AI", unless you plan on just copying someones code or have help from someone who knows exactly what their doing, that can be QUITE an accomplishment. I'm definitely not trying to discourage you from building the robot you want, but if you have never built one before and have no programing or electronics backround, a simple kit might be that best way to go. I can tell you from experience, and I can bet many others have had similar experiences my first robot was based on a 555 timer chip to avoid walls and obstacles, I had lots of mechanical knowledge, and lots of electronics knowledge and it still came out to be pretty much a failure. I wish now that I had started with some kind of BEAM kit to get me going in the right direction. But anyway thats just my personal experience, you don't have to listen to any of it, and at all costs don't be discouraged, like Edison said "If at first you don't succeed, try try again!"