Trial and error. Keep track of your time. Tell your customer they are getting a discount for your getting your name/work out for people to see. That way if you find your undercharging you don't set a president if he sends his friends over for a low dollar deal. When I started sign painting I worked for the experience at first. If I didn't come out too good on the price from time to time @ least I was gaining valuable experience. It's hard to put a price on experience. Once you build a good reputation the rewards will follow. Just my 2¢