The price for a commercial paint job can vary largely merely because of the location. While every painter must ultimately name her price for herself, there are a few aspects of every job that should always be taken into account so that you can do the job well, with appropriate quality supplies and be paid a fare wage for your labour.
Learn the average wage per hour for painters in your area. This will vary immensely and you may be competing with low quality, fly by night painters who will purchase the cheapest materials and do a bad job so that they can charge less to get the job.
Use this knowledge to decide how much you want to charge for your labour. Remember that this is your pay only and will not be supply costs. Do not forget to include your travel expenses, food and general living expenses when arriving at this sum. You may have to be willing to accept less if there is a great deal of competition in your area, so give yourself breathing room to pad when possible.
Decide how long it will take you to paint the room. Use past experiences with similar type rooms to get an idea of your average and estimate high. If you finish fast the customer will be pleasantly surprised, whereas if you fall behind schedule she will probably be upset.
Measure the square footage of the room according to the customer's needs. Do not forget to include the ceiling or floor if necessary.
Decide on your materials according to the customer's needs.
Use a paint estimator (available online) to get a general idea of how much paint you will need to buy to do the entire space. Add a gallon or two more depending on the size of the space and your skills for any mistakes.
Add up the total price of all the materials, including brushes, rollers and anything else you feel should be the customer's responsibility.
Add the sum of all the materials to the amount that you (and any assistants, helpers or employees) are to be paid, either by the job or by the hour. This will be your bid. If possible, decide on a lower amount that is still acceptable to give yourself leeway to bargain if necessary.
Faster is always better, as long as the quality remains high. Use a group of painters when doing large jobs to get them done well and fast.