The fee for a developmental editor typically depends on the scope of the project and the experience of the editor. It's best to contact a few editors and ask them for a quote based on the specific project you're working on.
Generally speaking, a single pass of editing can range from $1,000–$4,000 for a full-length book (up to 80,000 words), and $28–$50 per hour for shorter projects. An editor may also offer additional services such as line editing and copyediting, which will cost more.
When selecting an editor, it's important to consider their availability, experience, communication style, and publication history. A more experienced editor may charge more for their services, but their expertise and professionalism can be invaluable.