The USPTO provisional application filing fee is $65 for a micro entity, $130 for a small entity, and $260 for a large entity. There is no additional cost for submitting a non-electronic application (unlike a non-provisional utility application filed in paper). For complex patents, USPTO will charge you an additional $100, $200, and $400 (depending on the entity) for each additional 50 pages after the first 100 pages. You can check out all USPTO fees here.
Although you may qualify as a “micro-entity” and therefore only subject to a $65 filing fee, the law firm is including its own professional fees for preparing your provisional patent application. These fees could be from your attorney conducting a search, planning and drafting your application, filing the application, or even taking a phone call from you.
I commend you for seeking legal assistance for your provisional patent application as it is surely risky for inventors to complete the process on their own. Patenting your invention yourself can ultimately cost you more money if you make a simple mistake and have to then pay an attorney to clean up the mess. An attorney can also help you decide whether to apply for patent protection in foreign countries and guide you through the search process.