Citizenshipworks is free to use, which you can use to help you fill out your entire N-400 application, step by step. USCIS charges $725 to submit your N-400 Application for Naturalization.
You may be able to apply for a full fee waiver (I-912) or a reduced fee waiver (i-942). If USCIS approves your fee waiver, you will not have to pay the fee or only a partial fee. Learn more about the fee waiver at the USCIS Fee Waiver Guidance page: https://www.uscis.gov/feewaiver and here https://www.uscis.gov/i-942.