Hi Michelle,
The range for copyediting is pretty varied from what I gather
via a Google search. I'd suggest checking out how a few of these companies charge and what's included and just use that as a general guideline.
If you're not really doing the full copyediting, but more of a proofing then the rates are relatively lower (about half).
Based on the results I pulled up you're on the high end, but with the typing included in the service it certainly seems reasonable!
(Kudos for the client inquiry!

)