I do think it would depend for whom you are doing it, how you will be doing it (all by machine or machine then hand sewn down) and any other specifics.
When one of my quilting friends was unable to hand sew any more, I would sew her bindings down for free. But that was because we were friends and friends do for each other. But if this is going to be a side business then I would charge by the inch, and the charge would depend on what is being requested.