Guess it depends largely on the amount of work you are asking the contractors to do. They might give discount if you do more work, else, they charge by a minimum sum. Coz i did my kitchen and bathroom with the same contractor, 6 months later, decided to do come carpentry in the bathroom, but end up, my contractor charged at a higher rate as he said that its just 1 toilet cabinet base and the cabinet. But i still got them to do as they did a nice job previously.