The cost varies depending on how complicated the procedure might be due to some different factors. The procedure will probably be more complex than your first root canal treatment, because your restoration and filling material may need to be removed to accomplish the new procedure. In addition, your endodontist may need to spend extra time searching for unusual canal anatomy. Therefore, you can generally expect retreatment to cost more than the initial endodontic treatment.
While dental insurance may cover part or all of the cost for retreatment, some policies limit coverage to a single procedure on a tooth in a given period of time. Check with your employer or insurance company prior to treatment to be sure of your coverage.