Hiring a professional editor can be costly, but it can also be time-saving and effective. For a rough guideline, the Editorial Freelancers Association publishes a handy rates chart:
Developmental editing: At this stage I’ll help you consider the content of your work from a big-picture point of view. Who will find this interesting? What sub-genre does it fit into? How can the work be useful to its readers? Do you understand the plot line and know where it begins, the crisis, and the climax? Do you have an arc?
Line editing: If you’re clear on the audience and purpose for your work but need help getting your message across, this is where line editing (also called substantive editing) comes into play. I’ll look at the work closely, examining the linguistic and stylistic aspects as well as coherence and clarity. I can help you see where you have missed developing the story line or concept and where you need to add more sensory detail.
Proofreading: If your work feels complete and the content has been reviewed, proofreading is the step just before publication where the goal is to catch errors in grammar, usage, style (are you using Oxford commas?), layout, and typography (are you hyphenating long words as the text wraps?). Even for relatively small, simple projects, proofreading is essential if you don’t want to be surprised by typos.
My thanks to Tom Sullivan for his concise definitions. His website can be found here.