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.
I’m not sure why you said “ha” after Fiverr. I’ve been a proofreader (more like a line editor/copy editor, really) for 3.5 years and have worked on many book projects there and outside of Fiverr. (writerkerrie) … Yes, some editors on there are crap and I agree, but many of us ROCK! I only charge $5 for 1,500 words of text (I only receive $4) because I LOVE WHAT I DO and I love working from home and being able to homeschool my kids. I am also an author myself and want to help other authors by not overcharging them. I have received MANY book projects from authors who paid the rates you describe above and I ended up finding DOZENS more mistakes and GLARING errors that for $70/hour … well, they should have been caught. Please don’t write off Fiverr. Some of us care about what we do and we do it well and you can tell from my almost-1,600 great reviews that this is true. Don’t hire someone if their gig is written poorly and they barely have any reviews!
I implemented this way of working years ago and see how empowering it is for authors, and helpful for editors to boot. This doesn’t mean you’re asking your editor to do a free sample edit. (I discourage editors from doing this and caution authors who push for it as asking editors to work for free before you hire them may result in resentment.) Paying an editor for a few hours of their time to ascertain whether they’re a good fit for you is a great way to put your toe in the pond before submerging yourself. You can then see for yourself if the editor’s suggestions and comments resonate before moving forward with the whole project.
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. 
Join an organization, such as the National Association of Independent Editors and Writers (NAIEW) or ACES: The Society for Editing, to connect with others in the field. Publishing Professionals Network (PPN) lists jobs as well as other membership organizations. You can also subscribe to Editors Only newsletter to receive a directory of professional associations for editors, as well as current job openings.
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.
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.