I had someone send me a personal letter for editing that he wanted me to edit for free. He claimed he had a 45,000 word book to come later and he wanted to sample my editing services. When I declined, he sent me another email that suggested I was foolish to miss out on this opportunity and that another editor had similarly declined.
My experience is that people who want some free service or a massive discount based on generating goodwill for some future later work are generally unreliable as clients. After all, a business relationship goes both ways. Treat me with respect and get the best work I have to offer. If you are trying to get the most for the least, then you probably think I'm trying to give the least for the most. I don't have time for clients like that.
If you are looking for a book editor (and with more and more people self-publishing, this is very common) then there are certain steps you should take to ensure you are able to develop a good working relationship with your editor. Editing a book is more than just reading and correcting grammar. An editor will provide structural advice (or make structural changes), ensure consistency of style, provide fact-checking (if negotiated), and generally help you to ensure your book is the best it can be. Even if you are hoping to sell your manuscript to a publisher, you will need at least three polished chapters with your proposal.
The first thing to do is to narrow your search. Editors are specialists. My expertise is in academic writing and business writing. Book editors are even more specialized. If you are going to search the internet, use a search term such as "food book editor" or "technical book editor."
Better yet, use social media to help you find your editor. I'm a member of the Editorial Freelancers Association. Every day, I receive emails telling me about editing jobs that are available. Many are from people seeking book editors. If one of these fits my abilities, I can respond directly to the client. The client can choose between numerous qualified editors. Click here to post a job at the EFA site.
When you locate an editor, whether through the EFA or other means, provide that editor with a significant sample of the work that you are going to have them edit. This means several thousand words of a book, perhaps a full chapter. You are within your rights to ask for a sample edit (and the editor may or may not want to oblige). If the editor doesn't want to oblige, you need some other criteria to judge whether or not this person can do the job. If not a sample edit, get references, or recommendations.
For example, like many freelance workers, I have a profile on LinkedIn. You can see personal recommendations there. Moreover, I have a page on my website with recommendations.
Yes, you can negotiate prices. We all have posted prices, but especially for a longer job, like a book, the prices are going to vary. If your book is beautifully written and you have an excellent command of the English language, then the editor might be able to work quickly and complete a lot of pages quickly. If you are using English as a second language and the book is highly technical or requires a lot of fact checking, then the editor is going to want to be compensated for that time. That's why it's in both your advantage and the editor's advantage for you to provide a substantial sample of your writing for a sample edit.
You want an editor to give you his/her best. Begin by treating them with respect.