Digital PhotoPro magazine has published a case study showing how a professional photography job estimate is formulated. Read the article online here.
Industry best practice is to balance the client budget with where and how the image will be used, as well as for how long. Images are then licensed for Usage; i.e. Time, Territory and Media. When you pay a photographer you buy a license to use the images, you never actually buy the photo itself. Under Australian law copyright for commercial photography remains with the photographer.