Editing is a painstaking and time-consuming business. All edits require at least two thorough readings of the manuscript, plenty of thinking about the manuscript, and time devoted to annotating or editing the script and compiling a full set of editorial notes.
This means it may be more expensive than you were expecting.
As you can see from the case studies, these writers all thought it was worth the investment. Some went on to self-publish. Others have used the edit to improve their skills as a writer and are now embarking on their next book.