The cost for a developmental editor can vary greatly depending on the size and scope of the project. In general, developmental editors may charge a flat fee or an hourly rate. Factors that could play into the cost of a developmental editor include the genre of the project, the level of editing required, the editor's experience, and how complex the project is.
When considering the cost of a developmental editor, also consider that they provide a more comprehensive service than a copy editor. Developmental editors often make suggestions and insert comments to help strengthen characters, plot, and flow. This type of editing may take more time, and therefore, cost more than the traditional copy edit.
In addition to the cost of hiring a developmental editor, there may also be other costs associated with the project, such as cover design and formatting. Be sure to look at the entire package of services that the editor offers in order to get a full understanding of their fees.
Ultimately, it is up to you as the author to decide whether or not the cost of a developmental editor is worth it for your project. If you have any questions about the cost or services that a developmental editor offers, it is best to contact them directly to discuss it further.