Citizenshipworks is free to use to help you fill out your entire N-400 application, step by step. USCIS charges $680 to submit your N-400 Application for Naturalization.
You may be able to apply for a fee waiver (I-912). If USCIS approves your fee waiver, you will not have to pay the fee. Learn more about the fee waiver at the USCIS Fee Waiver Guidance page: https://www.uscis.gov/feewaiver