If the contractor is charging you $600 for a project why would you pay him for the item he bought at home depot or the tax he paid on the item?, Did you mean to say "if the contractor is charging you $600 for time on the project plus cost and tax on materials".....? I am a contractor and when I am charging $600 for a project, well that is the total price. You should be a little more clear when giving advise as your example does not pay attention to the details.
