I almost put this in the Editing part of the forum, but that seems to be geared for volunteer editors and such. The following is geared towards authors willing to pay for editors.
Michael J. Sullivan is a published author who gets 6-figure advances and I believe earns in the 7-figure number from his writing. So I listen to what he says. He participates on wattpad and offers advice.
Someone asked about hiring editors. This was his reply (which I thought was great or else I wouldn't have copied it here).
In general there are three kinds of edits:
* Structural - which focuses on "the big picture" - plot, character development, pacing and the like. It's a very subjective type of editing and can do as much harm as good. Generally speaking, I suggest people don't buy this type of editing - it's (a) very expensive (b) the people who do it well are working for publishers and can't freelance (c) hard to find a good person who has a lot of experience. - I recommend for this type of feedback the writer use beta groups and critique partners.
* Line editing - concentrates on the clarity of the prose. Are you repeating words? Are there two many pronouns? Is it reading too "choppy"? Is there a better word choice? Are the sentences passive? - You can certainly purchase this type of editing - and sometimes a copy editor provides this service.
* copy editing - concentrates on making sure the work follows standard conventions. It usually uses a style guide (like AP or Chicagon Manual of Style) to set the rules on things such as capitalization and hyphenation. Then it makes sure those rules are consistently applied. It also corrects any grammar issues such as tense, punctuation, and typo corrections.
My current editors don't ask for any money up front. But that is because we have a long established relationship. For new commissions usually 50% up front and 50% upon completion is standard.
Keep in mind, when he talks about beta readers, he uses several hundred.