There are usually three levels: proofreading, copy editing and line (or content) editing.
Proofreading: This is often confused with copy editing, but a proofreader is not an editor. Proofreading compares the copy edited manuscript to the printer’s proof to make sure that no errors were introduced during the publishing process. They look at the formatting, end-of-line breaks, paragraph and sentence spacing, and typographical errors. They also point out misspelled words or punctuation mistakes if they find any.
The suggested rate quoted by the Society for Editors and Proofreaders (SfEP) is £22.50 ($33.75) per hour. The average price per 1,000 words is £10 - £15 ($15 - $22.50).
Copy editing: A copy editor will check your work for spelling, punctuation, and grammar errors. They may also comment on continuity and do some minor rewriting if clarity is an issue. They don’t make significant changes or revisions.
The suggested rate quoted by the SfEP is £26.20 ($39.30) per hour. The average price per 1,000 words is £13 - £20 ($19.50 - $30).
Line editing: This includes all of the above and also points out things such as wordiness, overused words, repetitive sentence or paragraph structure. Line editing and copy editing have some similarities, but there is a difference. Line editing focuses more on paragraph structure, sentence flow, and word choice. They might also suggest some rewrites.
The suggested SfEP rate is £30.20 ($45.29) per hour. The average price per 1,000 is £18 - £25 ($27 - $37.49).
Anything over and above this is really into the realms of copywriting, which is a whole different ball game.
It is hard to work out how much can be achieved in an hour (10 pages?), so you may prefer to work with a price per 1,000 words or per page, which means the author can have a better idea of what the cost will be. In the copy editing world, one page equates to 250 words.
Ultimately you and the copy editor should feel you have a good rapport. They should always ask you for a sample of your manuscript and a word count before giving you a quote, and you should always ask them to edit a sample to see if you like the way they work.
You can find a copy of my blog in the Completely Novel newsletter. Completely Novel is a great site for those of you who are interested in self publishing. http://completelynovel.com/articles/how-much-does-book-editing-cost#comments