How much will it be to fix their citizen status?

Significantly more information is needed in order to determine the available and appropriate steps for your parents to achieve their immigration-related goals. Are you a U.S. citizen? Where are your parents located? What visa status, if any, do they have? When and where did they enter the U.S.? When they entered the U.S., were they lawfully admitted and inspected? Etc. It would be wise for you and your parents to work with an immigration attorney who, after learning all of the relevant facts, would be able to advise about immigration eligibilities, options and strategies, and would be able to let you know the amount of USCIS filing fees and related expenses, as well as attorney's fees, would be required. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge. [Note: This communication is intended as general information and not specific legal advice, and this communication does not create an attorney-client relationship.]