I'd quit writing before I'd pay anyone for an appraisal, or use a book doctor. I am the book doctor. If you can write, you don't need a book doctor, and if you need a book doctor, you can't write.
The only appraisal service or book doctor any writer should need is the freebie you get when submitting something.
If your novel isn't close enough to professional quality to make a good agent or editor tell you what needs changed, then you need to learn how to make it so.
Money should flow to the writer, not away, and books should be written by the writer, not by anyone else.
James, I think the first part of your comment contradicts the part I've put into bold.
I agree that there's no point using an editor if you get your editor to improve your work for you, and that's that: but if you use an editorial agency which tutors you through the process of revising and improving your work (which is what the better ones do) then you're learning skills which you can apply to future titles: you're not just revising the book in front of you, you're learning how to make all your work better. And that's when it's worth employing a good editorial consultancy.
What James said is harsh, but true.
The only time I'd use a paid editor is if and when I have to self-publish a big novel, and then only for line edits and proofreading. By that point in the process, my beta readers and I should have already dealt with structural errors.
I've read many books which have been through beta-reading, but which are rife with structural errors (and others too). Beta readers aren't necessarily the answer.