Some authors budget very unrealistically, perhaps just a couple hundred dollars. (I’m not saying this is necessarily true in your case, but I have encountered it.) In such cases, I find myself wondering whom they are imagining should bridge the gap between what they are comfortable paying and what the editor needs to earn in order to pay expenses and support him- or herself.
If however the editor is going to disassemble the book act by act, scene by scene, beat by beat, and show technically in story structure terms what needs to be changed, then the editor is spending far more time in analysis and constructing the demonstration of such to the client. Of course, this really depends on what service a client wants as well.
As for multitasking, believe it or not, I try to schedule so that I am only working actively on one book at a time. Research keeps showing that “multitasking” is actually less efficient, because of the time lost to leaving behind one task to restart another. (Think of it this way: If you take on two clients at once for work that would take one week each with your full attention, either you can tell both clients the work will be done in three weeks because you’ll be alternating between them, or tell one client it will be done in one week and one that it will be done in two weeks, because you’ll complete one before you start on the other. Which of those makes more sense?) Obviously, with back-and-forth communications, sometimes complete “monotasking” is impossible, but I think it should be considered the ideal.
It’s best, if you decide to hire a freelancer, to figure out what the going rate is for the service you need, and expect to pay that amount. Otherwise it may turn out to be a waste of money, because the quality of the service will suffer. Sometimes there are payment plans to help with high costs! But it’s really a good investment to pay industry standard for editorial services; I agree with Shayla.
Having your manuscript reviewed by a professional book editor is a rewarding experience. In addition to the improvements to your manuscript, you will also benefit from the feedback on your writing that only a finely tuned professional editor can provide. Writers often comment on the lifelong benefits enjoyed from their very first editing experience. If a writer wants to ensure they are creating an impactful and enjoyable reading experience for their readers, then editing is a no-brainer. Remember nothing is more damaging to your author brand that publishing a book with embarrassing typos and grammatical errors. Read more.
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Regarding your last question, I know of no editor who gives a money-back guarantee for occasional errors that happen to slip through or for a different opinion by a later editor. I’m afraid such things are just the nature of the publishing beast. But by having a shorter portion edited first, you will get a decent feel for whether someone is the right editor for you, and will be able to be confident in his or her work on your entire book.