There is no definitive answer to this question. It will depend on the editor in question, and the writer's budget.
The National Union of Journalists (NUJ) suggest the following hourly rates for the professional editing of books:
However, in reality, rates are not likely to reflect these figures and are usually less. It's also worth noting that the above encompasses the editing of different types of books. As each type will require its own specific skill set, rates may differ.
Professional editors may charge by the hour, the day, per number of words, or for the whole job, and because it is not always possible for an editor to determine at the outset how much work will be required, the writer should be prepared for some fee negotiation if a manuscript becomes more difficult to edit as it progresses.