I don't quilt for hire, but I do longarm comfort quilts for various guilds. I have been given yardage for backs (rather than backs already constructed). When that happens, I sometimes return the quilt and ask for one that is "ready to go", or sometimes I piece it and then don't offer to do another for that group. Piecing the back is one of my least favorite things. I know someone needs to do it, but the service I'm providing (for free) is longarming, not piecing.
Another problem is when the quilt arrives without batting, and I am expected to provide it. Sometimes I do, but again... I'm probably not going to offer to do more quilts for that group.
To address the original question, I have seen longarming contracts with everything but the kitchen sink listed, along with prices. Usually the longarmer has a set hourly price for work that is not done on the longarm.