I got your mail. I've been racking my brains thinking about what to say and I really am not sure, as I am new to this. Try looking up societies/groups for copy-editors and writers in the USA, as they probably give guide price lists and may be able to help you more.
Here in England it is the norm to charge per hour for editing, but I am not sure that this is the case for writing, where it is more likely to be per piece.
Hope this ramble helps, sorry I can't be more use.