Word count
No matter what method they use to determine project cost, most editors use the word count as the starting point for their calculations. Some editors charge a fixed rate for a certain number of words (e.g., $20 per 1,000 words). Others will charge an hourly rate or a flat project fee; however, the word count is still part of their calculation. Most editors I know track their time when working on projects. Doing so helps them determine the average number of words they can edit per hour, which in turn helps them estimate how many hours a project will take them.
Document type
These days, most editing work is done in MS Word using the Track Changes and Commenting functions. Working in a format outside of Word (e.g., a Google Doc, PDF, or hard copy) usually takes more time; consequently, the fee for those jobs may be higher.
Specialized knowledge
Do you need someone who knows the difference between the gastrocnemius, the soleus, and the plantaris, and can describe what they do? Or maybe you need someone familiar with the phenomenon of column-swallowers in medieval churches? Are you writing about constitutional law? In these scenarios, you’d want an editor who’s a subject-area expert, and you’ll likely pay more for their specialized knowledge.
Deadline and existing project schedules
Many freelance editors have projects scheduled months in advance. That doesn’t necessarily mean that an editor won’t be able to accommodate your project’s tight deadline. But they’ll charge you more than their normal fee since taking on your project may mean working overtime to complete it by your deadline.
Manuscript stage
By this, I mean is it an early draft or has it been edited already (whether professionally or by the author)? How much work does it need? If you send an editor a 250-page, single-spaced first draft that you haven’t reviewed since you finished it last week, well…they’ll likely charge you more to work on that than they would a draft that you’ve revised at least once and formatted so it’s easier to read. An editor may ask to see a representative sample of your work before giving you a quote so they can assess its stage and get a feeling for how much intervention it may need.
Number of passes
The number of editorial passes you want an editor to do—that is, the number of times they read a manuscript—will impact your cost. Be sure to ask how many passes an editor’s quote includes. Some editors’ fixed project fees include multiple passes, while others include only one pass.
The type of service you request
There are several different types of editing services available, and different editors provide different combinations of these services. The important thing to remember is each one of these services is distinct, and the time they take to complete (and, therefore, the cost associated with them) varies.