There is no cost to you. Simply tell the grantor you wish not to serve as POA. She will then have to revoke and prepare a new POA leaving you off at her expense.
I agree with Mr. Brinkmier, however ... if the principal's health is such that he or she is unable to remove you (or they are unwilling to remove you), someone needs to pay an attorney to prepare a "resignation" of agent so banks and others will know that the backup is duly-empowered to act as Agent under the POA.
Mr. Huddleston is an Ohio-Certified Specialist in Estate Planning, Trust & Probate Law, with offices in Columbus and Dayton, serving client families and private business owners throughout Ohio. He may be contacted directly by phone toll-free at 888.488.7878 or by email CLH@HUDDLAW.COM. Mr. Huddleston responds to Avvo questions as a public service to help educate and provide general guidance to questioners, but his responses are not legal advice and do not create an attorney-client relationship.