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 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.
In addition to the individual editors listed above, the Reedsy marketplace is full of vetted professional editors for every editing level and genre. They screen their editors and only allow the best of the best to be listed on their website. I'm a BIG fan of them because they have made the process easy for me to find the best book editors, book cover designers, and even writers to help me with my projects.  Talk about awesome!
Next, it is possible to follow the road to traditional publication without hiring your own editor, with editing being done at the publisher’s expense. However, you have to be a very good writer and self-editor to get on that road, and what you write has to fit into a very marketable niche. You also need to be very patient and willing to weather plenty of rejection along the way. Don’t jump too quickly to the conclusion that this option is closed to you. Do your research carefully. There may be a small press that is just right for your book and that will be happy to help you make it the best it can be while paying YOU rather than making you pay them.
I have over 30 years' experience in the publishing/writing industry, working as everything from an in-house editor to an investigative reporter to a novelist to a medical/technical writer to a screenwriter. I've been an independent contractor for 24 years working as a freelance editor (developmental and line editing), writer (fiction and nonfiction), ghostwriter, and copy editor.
I'd suggest working through each change so you can learn for next time, rather than just clicking Accept All on the Track Changes version. This will take time, but it's well worth it. I usually accept 80-90% of my editor's changes and the manuscript is improved considerably by the experience. After the line edits are updated, I send the manuscript to a proofreader for the final check before publication.
The price you’re going to pay an editor depends on a lot of different factors. You may just want an editor to proofread your book to catch minor errors and typos. Or you may want to work with an editor to help you overhaul your book. Getting familiar with the kind of editing options available will help you choose the right editor. Typical book editing services are:

I totally agree that you need an editor. I disagree that it has to be someone else. Editing is an entirely different skill than say writing, but it can be learnt (editing covers more than writing, at least in terms of publishing). Editing your own work is harder than editing someone else’s, but it can be done. Also, learning to edit is hard and takes some time, regardless if you are editing for others or yourself. And it is interesting that this apparent need for an editor happens within writing. Other creative disciplines, music, painting, photography, theater, etc, the idea… Read more »
Thank you, Blake. I am past the point of undercooked. I’ve had beta readers, I’ve rewritten dozens of times, had an editor look over my MS twice, and believe I’m ready to take that leap. Unfortunately, and this wasn’t mentioned, most writers will never be satisfied with their work ‘as is’. I tinker and add. It’s difficult to find the balance between making it better, and messing with the core integrity. Even JK Rowlings is known to do this in her head when she’s reading one of her novels in front of an audience. This fear has held me back for years, despite my three years experience as a freelance writer for a local magazine. I’m scared I’ll miss a comma and it’ll be thrown in the slush pile.
I cannot speak for all editors, of course, but I know I and many of us would be open to editing just part of your book at first, say, a particular chapter or two that you sense needs a lot of work. That is often enough to show you mechanical errors that you tend to make repeatedly. You can then go through the entire manuscript yourself looking for as many of the same kind of errors as possible. You will sharpen your basic writing skills in the process, and it will pay off in all future projects.
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