Why is Book Editing Necessary and Often Expensive?

I’ve just been reading about self-published authors’ points of view on the necessity or editing their book, and was surprised to find out experienced authors do value professional book editing services – once they’ve experienced it. But how do we achieve affordable book editing rates? Obviously I have been in both the writer and the editor’s seat, so I know how much time it takes to edit to a good result and how at times you need to “switch brains”. With the luxury of time,…