Add the cost of all supplies you use, cost of your building/store, your advertising. Add what you think your hourly time is worth. If it is a hard piece or the customer is a problem, add in a inconvenience factor.
As you do this, you will get a feel for what repairs cost you. You can make your own price list to keep from refiguring each time.
With experience, you will get faster - the time you take goes down, but your experience should produce better work. It should even out.