I charge $.25 per linear inch for both hand and machine binding. Yes, I charge the same for machine because in my opinion, machine binding is more of a pain to do and I would rather do by hand. I prefer my customers to just give me their binding fabric and I take it from there, because most of them press it in half first and that's yet another pain. I have to press and starch their fold out before I can use it.
So, to answer your question, if the quilt was 40" by 50", I would charge $45.00. I have plenty of samples at the LQS for anyone to look at and handle, and lots of references.
I agree with Kalama - price seems awfully low.