I'm a longarm quilter. I keep it simple. 2.5 cents a square inch for edge to edge and 5 cents an inch for custom quilting. The price for custom quilting is too low in my opinion but thankfully I don't get too many of those, they take ten times as long to do and I have to drag out a myriad of tools as well. For binding, which is rare, I charge 25 cents a linear inch, that is machine sewn to the front and hand stitched to the back. My customers know my prices and they either accept them or take their business elsewhere. Another longarmer in town charges 1.9 cents a square inch but it takes her months to get a quilt done and I usually have a 3-5 day turn around.
Remember you time is worth something as well as your skill. Don't sell yourself short, if they could make it themselves they would! Have your prices SET ahead of time, even print them out to hand to your customer so they know you are not just talking off the cuff. This helps you to be more professional also. Hope this helps.