The track system alone is going to cost $300.00 and that does include the motors.
You might want to rethink the use of a tracked-based skid-steer robot, especially if the weight is going to be over a few pounds or so; it will tear the hell out of your flooring. My wife and I have a roomba that we use fairly frequently; along the "wall" edges and other areas that it follows constantly, the carpet is fairly well mashed down (we don't really give a damn - I'm just using this as an illustration of what can happen when using a relatively light-weight wheeled robot; just think of what a tracked one will do over time).
Using an electric wheelchair as a base, or something similar with wide tires might be a better choice (and if you shop carefully, it may even cost less).
That said - here are a couple of interesting links (and granted, they do use a "roomba" as the base - but that is infinitely better than what a tracked base would do to your floors):http://www.instructables.com/id/Belvedere-A-Butler-Robot/http://www.instructables.com/id/Open-Source-iRobot-Butler-OSIRB/
Also, you might be interested in finding a copy of the following book; while a bit out-dated, some of the mechanicals and other ideas are still interesting:http://www.amazon.com/Android-design-Practical-approaches-builders/dp/0810451921