If you have the time and energy to do all the filling yourself, your only costs will be the certification stamps the court requires of any filed documents. However, the process can require numerous trips (and long waiting lines) at the courthouse. If you plan on doing the process by yourself, consider buying a book or do-it-yourself kit such as Divorce Yourself: The National No-Fault Divorce Kit, by Daniel Sitarz, which contains all legal forms, checklists, and courtroom guidelines you need.
A divorce company will do the whole process for a flat fee, and avoid you frequent trips to court by taking your testimony, compiling all necessary paperwork, and filing all forms for you.