Read tips from such sites as the following:
http://digital-photography-school.com
www.betterphotography.com
These sites will teach you how to compute your overhead expenses like the worth of your camera and gears, how to price according to your competitors, how much your clients can afford, will you scare off potential clients because you charge too low or too high, etc. etc.
There are many sites that can guide you how to price your work but it all boils down to your worth as an artist after you deduct all of the necessary overhead expenses.