Hi Andrea. In my experience it depends on the depth of the projects and the amount of projects. If you prefer someone else to handle the entire rehab process so that you can focus on other tasks at hand then a good GC or project manager is extremely valuable. The cost varies a lot depending on the manager and how they price things also vary.
The fee depends if you are hiring a full time project manager that would basically be like an employee or if you are looking for a GC that manages their own projects. Basically a GC will incorporate their costs into the total bid for the project, a project manager a lot of times would be an employee of a larger construction company, a builder, or investors that take on a lot of projects. Most GC's will give you a price and a payment schedule, most payments are due after a specified amount of work is completed. For instance all demolition is done, all rough plumbing is done, all drywall is done, it really depends how you work it out with your contractor.