Unfortunately, no one can give the person who asked this question online the other day a specific, accurate answer without knowing more about what is required. Asking for a price to edit a 600-page manuscript is nowhere near enough information. With that said, here are a couple of points I suggested the author consider.
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First, I'm happy to see that you recognize your need for an editor for your work. No writer can know how to edit simply by having written a lot of words. Writing requires one skill-set (or, some would argue, no skill-set at all, but let's not get into that); editing requires another. Without learning what editing involves—and then learning what good editing entails—no one can sufficiently edit anything but the most rudimentary piece of writing with the hopes of improving its readability.
Second (and here's where things get dicey), few editors can do all types of editing. And editing does come in a plethora of flavors. Not butter brickle, unfortunately, but things that can be just as enticing, if less indulgent. Depending upon who's doing the defining, an editor can be a proofreader (checking for typographical errors and basic grammatical and punctuation mistakes). He can be a fact-checker, a copy editor (reviewing the author's copy for readability, proper syntax, and the like), a conceptual or substantive editor (also at times referred to as a content editor, checking a piece for various flaws in logic), or a developmental editor (reviewing how the story is put together and marking various recommendations from that point-of-view).
An editor can also be a structural editor (specializing in structural issues concerned with a story's readability and involving the use of flashbacks, flash-forwards, linear chronology, and even when and where to break a story into chapters and other divisions), a line editor (concentrating on the flow of the prose and pointing out any awkward phrasing, etc.), or any combination of the above.
Clearly, not all editing involves the same amount of work, which editors gauge by the amount of time they'll need to spend on a project to make it right. The best, most accurate way for an editor to determine that is to make a good read-through of the property and estimate how much needs to be fixed and how long that will take. The second best way, sans a read-through, is by checking the word count. That's far less accurate and desirable than a read-through, though, because the same number of words written by an experienced writer can take a quarter the time to edit as a property written by a novice attending his first rodeo. Still, sometimes an editor has to go out on a limb and make a judgment call, for better or worse.
With all this said, you've probably guessed by now that giving a page count as a basis for determining the cost of a good editing job is useless. The accurate estimate of the cost of editing depends upon the type of editing you want done (or all editing as outlined above), the amount of work involved in improving your writing within the realm of that specific editor's sphere of knowledge, and the amount of time the editor estimates the job will take depending upon the number of words in your "600 pages."
Now, taking all that into account, let's make the assumption that you're asking about hiring an editor because you want to see your book in print. Most likely, you'd like to land a contract with a conventional publisher such as Doubleday, St. Martin's Press, or Penguin. If that's the case, you should understand that no one editor or even a combination of editors may be enough to accomplish the task. To do that, you may need some other kind of genie altogether.
Beyond the scope of editing, if a work is really "raw" (see how nice I'm being here?), it might require substantial reworking or rewriting or even new writing to fill in any gaping holes that the offer may have left behind. That calls for the services of not only an editor but also a book doctor. He's someone who can do all the editing and round out the story by adding or subtracting material wherever it's required (with the author's approval, of course). That's what I do for my clients because I enjoy knowing everything there is to know about writing, exercising my knowledge to my client's benefit, and creating an absolutely perfect book that's primed and ready for sale to a conventional publisher. That's what excites me the most—the challenge of taking a rough manuscript through all the stages necessary to make it irresistible to publishers. No single editor can claim that same type of "rush."
So, with all that said, where do you start? Here are my suggestions:
Put out some feelers. Ask some published authors whom they used to help get their books in print. Read suggestions on sites such as this. And send out a few e-mails. Then, carefully evaluate what each of the respondents says about what he or she can provide you toward reaching your publication goals. Doing so won't be as quick as asking a bunch of perfect strangers for a price online, but I guarantee it will be more accurate.
Smoke if you've got 'em.
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D. J. Herda is author of the new eBook series of writing advice, About Writing Right, available at Amazon and at fine booksellers everywhere.