They are free to charge whatever they want for their services. However, competitive market forces tend to keep their prices down, just as the cost of doing business tends to force their fees up. A good, well-run business tends to be busier and able to charge more then a poorly run business. Photography charges will vary in different parts of the nation. Generally, photographers in the largest cities are able to command the highest fees.
Many commercial photographers charge for time plus materials and expenses.
The guide for comparison is usually the day rate. This is what a photographer will charge for a full day of shooting. On top of these time charges will be the expense for film and processing. In addition some photographers may have a usage fee that will either restrict how you can use the photos or make additional charges for other types of use. It can cost you a lot or it can cost you a little depending on the photographer.