The team at Ebook Launch was highly recommended by a publishing website I trust, and I turned to them for helping me format my ebook. They have been wonderful to work with! The turnaround was unexpectedly fast and I got incredible value for my money. But more importantly, Adrian gave my book the same care I would as an author. He offered suggestions that improved its appearance and provided a beautiful, clean format. I also appreciated his patience and friendliness. If you want great service at a great price, Ebook Launch is the way to go.
I do developmental editing mostly, so I charge a flat rate to read once and build an action plan, half before and half afterwards. Then we contract to work through the plan, where the bulk of the editing takes place and for this I charge 25% up front, 50% after the developmental edit, and 25% on completion of the final proof. I find that authors appreciate the spread payments and the transparency this brings.
I’m a first time writer. Today I’m about 60% done with a “novel” (a thinly disguised autobiographical account of finding romance and starting a family during the Viet Nam war). I believe that hiring a developmental editor at this time would help me complete the work in a better ‘voice’, ultimately saving time and money. The alternative would be to finish the work as this column advises before submitting for edit.
Book editing is both subjective and not. There are rules, and yet the rules of writing are often intentionally broken. You want to find someone who knows the rules, who will fix the writing, and who’s flexible enough to know that voice is sacrosanct, and that editing out colloquialisms, slang, and humor is a no-no (unless it’s overdone or not working, that is).
At first, I didn't want to add Fiverr to this list because there are a LOT of editors on Fiverr that aren't exactly a good investment.  But, that doesn't mean there aren't amazing opportunities out there.  Editors like Kerrie McLoughlin, have done an amazing job with their Editor Fiver gigs.  However, if you do decide to go through Fiverr, then ABSOLUTELY use my test to ensure you aren't hiring someone who isn't qualified.

Contact the publisher directly by phone or email. The publisher’s website and contact information will likely be printed at the top or bottom of the copyright page. If it isn’t, you can look it up online. Contact the publisher by calling or emailing them to ask about a book’s editor. Publishers keep detailed records on each of their texts, so if anyone would be able to find a book’s editor, it would be them.[6]
What an editor does is discover your characters, your situations, and your images without seeing any of the creative process that brought them to life. Where you might see all the crossings-out and labors, all the accidents and decisions, the editor sees only a page. This is the clarity you need, and you can never achieve it for your own writing, simply because you envisioned it first. The editor will tell you what an attentive, an educated, and, most importantly, a new reader will experience while reading your book.

That’s a great point about other disciplines. Why should writing require some outside arbiter? Is it because these other disciplines consist in crafts that require significant training, whereas anyone who’s literate can start writing a book at any time (and therefore, the threshold for entering the medium is much lower)? I do hope you’re right about learning self-editing skills. I actually just made a YouTube video about how to self-edit:
First, decide whether your time or your money is worth more to you right now. You can greatly reduce the amount of money your project will cost by investing more time, and vice versa. If time is no object, you may want to spend some hours in the library reading style manuals and going through your manuscript with a fine-toothed comb so that when you do hire an editor, (s)he will be able to zip through it quickly. If you end up deciding that’s more effort than you want to invest, then you will at least find the monetary cost of hiring someone to do it for you a less bitter pill to swallow.
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.