Some divorce mediators have a set fee for all sessions, from inception to agreement. Others, like lawyers, charge an hourly rate. Be sure this information is provided to you and your spouse before you hire a mediator, and that it is in writing. Make sure you and your spouse have agreed, in writing, on how the mediator's fees will be paid. Like some lawyers, some mediators require an advance fee to be paid before any work is undertaken.
In other words, you may be paying three people, but the total fees may be less than if you had paid two, or if you had proceeded with litigation.