Thanks to all of you for your replies. I have to say I wish that all tipping were abolished and that we simply paid people well for their hard work. If that were the case, my question would be irrelevant. I have hired a quilter three times, and each time she charged in the $150-$185 range for a bed-sized quilt and for machine sewing the binding. (I do the handwork on the binding.) I really don't know how much time it takes, but I do know the process and how expensive the machines are. The prices I've been charged seem like a bargain to me.
When I learned to piece last summer, I was disappointed to learn that I would be hard pressed to machine quilt it on my sewing machine (bed-sized, that is). It took me a long time to decide what to do with that first completed quilt top. I worried that if I hired someone to do it, I wouldn't feel the satisfaction of having completed it myself. Hand quilting is really out for me because I have arthritis in my hands (and everywhere else for that matter). When I decided to hire a long arm quilter, it really set me free to love quilting, and I've been very prolific. I've completed nine quilts in the past 15 months. I've been bitten badly by the bug.
I have the utmost appreciation for the work these women have done for me, and I credit them on my quilt labels and in my quilt journal. As for tipping, I think I'll put the matter to bed now (no pun intended), and just get on with loving what I'm doing. Thanks again.