Not sure where you are located.
Around here the easiest way is to charge by the hour. Make sure they understand that it is also for getting materials, going to the dump, etc.
Usually I figure how long it will take and charged my hourly rate, which included my wages plus the expenses of running the business and insurance.http://www.contractortalk.com/f16/pr...uccess-122452/
Contractor talk is a great forum for those in the business.