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.
Definitely not unreasonable, Frank. However, you have a couple of options. Rather than pay for a full developmental edit on what you have written so far, you can ask the editor to do a manuscript assessment – which includes a detailed written report. An MA is usually done for a fixed price based on the word count. Alternatively, you could ask the editor to do a sample/developmental edit on one chapter only. Using track changes, they can go through and make edits and comments. It’s good that you are already thinking of hiring an editor before you complete the MS. It will help guide you with the other 40% and revisit what you’ve written so far, so that once you are finished, the MS will be in much better shape for the next stage of editing.
For editors who are charging pennies for what should be hours and hours and hours worth of work: I am saddened that you would ruin the market for those who have the expertise and ability to charge what it’s worth. You see, an author whose book needs a whole lot of help will see your rock-bottom prices and think that’s the norm. But the reality? That’s not normal. It’s not normal to charge sweat shop rates for something that requires top-notch quality.
Great advice, especially the painting analogy (clever!). I also enjoy your YouTube videos, which I found out about through Publishing Perspectives ((nice!). http://publishingperspectives.com/2015/08/ready-for-your-close-up-what-youtube-can-do-for-writers/ I agree that most developmental editors are more like therapists than surgeons, but there are book doctors out there who will essentially rewrite your manuscript (they’re one step away from ghostwriters, I feel). Also, I don’t actually offer sample edits, but this has nothing to do with pedigree. It’s more that I’d rather not mark-up a manuscript unless I’ve read a good part of it (at least 50 pages). I believe that most copyeditors offer sample edits,… Read more »

The job board is NOT a big reason to join the Den, especially if you’re a new writer. It’s a higher-end board that doesn’t post any low-priced gigs, and is a good resource for mid-career writers. But in general, the board is a real sidelight to the point of joining the Den, which is to access 300 hours of training+, 24/7 forums for support, live events where you can ask questions of experts, and more.

As a freelance editor, I would like to express my strong disagreement with the idea that a freelance editor must work as an employee of a publisher for three years to be any good. In my field, spirituality, many publishers have gone to having most editorial tasks beyond acquisitions performed by freelancers, so the in-house positions being held out here as a prerequisite for freelancing simply don’t exist; many editors must cut our teeth as freelancers. It also occurs to me that self-published authors usually bristle at the idea that they are not “real” authors unless their work is accepted by a commercial publishing house. Is it fair to apply this standard to the people they propose to employ as editors?

From your comment, you may be in search of a manuscript critique, where you pay a professional to essentially conduct a beta read with the added benefit of their experience and knowledge about the craft. Then they’ll give you a few pages’ worth of considerations. When I don’t have the time for such critiques (and I only critique nonfiction), I refer my clients to andilit.com.

If you know you are sensitive to changes, or that you only want edits that correct what’s wrong (and not subjective edits beyond that), tell the editor so. This kind of information is invaluable, and will prevent frustrations and misunderstandings. That said, don’t suggest this if you’re open to your work being better. Editors will bring your work to a whole other level if you’re able to loosen your grip on any notion that your creative expression is under attack when an editor makes changes to your work. The most successful authors are edited multiple times over.
About 25% of my (fiction) editing work is direct with authors, and the rest is split between two publishers. It my be of interest to note that these rates are close to what publishers pay their editors. That is to say, if you, as an author, were to traditionally publish your book, you would be paying this amount via royalties. Creating a professional product via self-publishing is not cheap, the costs (and income!) just shift from the publisher to the author.
The U.S. Bureau of Labor Statistics states that employment of editors is expected to have little change up to 2026, as print media continues to face strong pressure from online publications. Editors who have adapted to online media and are comfortable writing for and working with a variety of electronic and digital tools will have an advantage in finding work. 
Whether you’re just starting out, or you’ve been writing for some time, it’s no secret that the most successful writers specialize in a niche. Why? Focusing on a niche helps you become an expert in that area, write better content, ask better questions, and know where to find sources and research. The best editors typically specialize in one or a few niches for similar reasons.
Unlike most other basic “proofreading” packages, ours includes copy-editing where needed, light revision, rewriting and improved word choice, as well as a one page summary of some big picture issues that can be improved. If we see something that looks bad, we’ll fix it – but we won’t wrack our brains trying to solve bigger issues like plotting, character motivation, story architecture, etc.
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.