FFL holders are in business, which means they don’t give away their services for free. Different FFL’s will have different rates for different services. These rates vary based on local demand and competition, and the amount of work that goes into the service.
The most common service FFL holders offer is that of transferring guns. Because all online gun purchases have to go through an FFL for a background check, most FFL holders will handle those transfers for a fee. Some charge a flat rate, others by the gun. Some charge more for guns that might come with extra state paperwork like handguns. Some don’t handle transfers at all. In states that prohibit face to face private sales, FFL’s also perform background checks for those transfers and charge a variable fee for it.