Assess the cost of the materials you will need to complete the job. The condition of the house and the desires of the customer can cause great fluctuations in this amount. If the house has not been painted in many years, it will require more materials than a house that was recently painted. Bring a list of what you will need to complete the job to the store with you, and get a rough idea of what all the items on your list will cost.
Use your personal experience to determine how much of the house you can paint in an hour and then how many hours it will take you to complete the house. Next, you need to determine what your time is worth. This will be influenced by your experience, the current market price and your impression of the quality of work the customer desires. Multiply the amount of time it will take you to finish the house by your hourly wage, to calculate the cost of labor.
Add together the cost of labor and materials, to reach total estimate for the job. Now, reassess the number by asking yourself if this is what you would expect to pay if this were your house.