The last few times I've been to writers group, the issue of editing services has been broached. The biggest question on everyone's mind is price, but price is a difficult metric when it comes to editing, because the services provided under this broad heading are fairly diverse. Here is a breakdown of several different types of edits which may be purchased and what a writer might expect from each.
Copyediting is often referred to as line editing, and is a light edit which is often intended to be the final edit before publication. The editor commits to fix any errors in spelling, grammar and punctuation. Since this tends to be the very last edit after all other significant issues have been resolved, copyediting is often the least expensive edit.
Proofreading is similar to copyediting, and tends to be the term applied to shorter works, particularly business writing.
3. Substantive Editing
Substantive editing (or rewrite) occurs when the structure and organization of the work is under scrutiny. In this form of editing, chapters, scenes, paragraphs and even sentences are tightened and clarified. This is a much more detailed process than line editing, but is not as complete a process as is developmental editing, since a substantive edit still restricts itself to looking at the actual prose -- rather than critiquing the story. Sometimes there may be some cross-over here.
4. Developmental Editing
Developmental editing is big picture editing. This is where an editor looks into the strengths and weaknesses of the story itself, and the overall organization or the book is considered. Issues such as pacing, character and/or plot development, as well as dialogue (among others) would be considered in a developmental edit. Coherence and logic will also be considered, with the possibility of text being rearranged, removed or added in places. The overall question being asked is, was the book readable and enjoyable?
This stage of editing can be a lengthy process with some back and forth between editor and author. It is wise to negotiate the timeframe for turnaround of such an edit. Because developmental editing is a lengthy and involved process, this is generally the most expensive form of professional editing.
While this is not a comprehensive list of all forms of editing and while there can be overlap within the categories, in general terms, these are the most common types of professional edits a manuscript may be subject to. You may also chose a formatting edit, a reference edit, a mechanical (style -- MLA) edit, or a simple peer review. The length of turnaround will vary based upon the type of edit and the workload of the editor, as will the cost of the job.
Editors can be found through word of mouth, local job boards, online advertising and through professional writer's and editor's guilds or conferences.
Happy writing, and Good luck!