I have created a list of equipment and supplies you will need to build you own robot. I have attached an excel file that list all the items and website links for the supplies. The total cost for the basic robot is just a little over a $100, but this does not include shipping. Some things you may want to consider going to a store nearby that sells these products, like the balsa which can normally be found in a hobby store or craft store.
I also added some optional items. I suggest holding off, on getting the battery for they can be a little expensive and the one listed doesn't even come with a charger. I suggest just getting a 5 - 6 volt source and run a wire, because the robot won't be walking very far, very fast. Finally I would suggest getting the accelerometer to help the robot balance itself, but it may be a little hard for someone that doesn't have much experience in programming.