Sarint

I am fairly new to quilting, started with baby quilts and now making twin or full size. As these are too big to do on my home machine, I started using a long arm quilting service. I have found a wonderful lady who does LAQ out of her home and she does excellent work. I do my own binding to save money, but I think she is working incredibly below the average market price. I don't have a good feel for what I should be paying, but most places I have checked out online won't touch a quilt of any size for less than $50 and then you pay extra for the batting and thread, by the square inch, etc. She has charged me $20 to $30 for the smaller quilts and has never charged me over $36 even for a full sized quilt. I am not asking for special stitching, just meandering stitch or simple all over machine patterns. She charges nothing extra for the batting or thread. I tactfully mentioned that she had great prices and she said well it only took her a couple of hours to do one and shrugged it off. I actually feel guilty that she is making so little profit for her work. Would it be inappropriate to tip her for a job well done? It is common to do so?