You can avoid this problem in the future by developing a written contract that does the calculations for the client so there is a total cost at the bottom. Not everyone is good at math, especially where inches and cents are involved! The written contract can have a space for quilt top size (both in inches and in total square inches), cost per square inch, and then calculated price. Include a blank space for price if side leaders need to be added (and put that $15 in there so they know that's what will be added if side leaders are needed).
I attended a Jamie Wallen workshop. He said that the first thing he does with every quilt that comes in is this: he folds the top into quarters, folds the backing into quarters, then places the top on the backing. This is a quick way to tell immediately if the back is going to need side leaders (and also can tell you immediately if the top is wonky and will need extra loving care on the frame).