I have yet to charge for a quilt. I have made "Gifts" for many and also have had people buy the fabric for smaller items and I have sewn them at no charge. I do sterling silver jewelry and have always used 3X formula on materials. This seems to cover my labor unless I am working on something very intricate. I also do custom stained glass and have found that the most important component is the communication between my customer and myself. Based on the complexity of the project I charge per piece of glass ($1.00) or by the sq foot ($75-$125). I also let them know that although they may say they want a bluish-purple....the final glass choice is mine as the artist, unless they want to go to a store and buy the glass at retail. I would do the same in quilting. I have had long-armers charge me $300 to over $500 on kings for the quilting. Add material cost for a king and your time and I won't sell one of my finished kings for under $1000. Look at what a finished hand made quilt costs, I live near Lancaster, Pa so I can check these prices on a regular basis. Remember people that don't do hand crafts: knitting, sewing....have no concept of your time and the work that goes into a project from design to finished item. Do not short change yourself, if you were a business what would you charge....electric, maintenance on the machines and so on. I know this is a lot but remember you are now opening a door that once you walk through you have committed.
Good Luck!