Quick answer: An FFL costs $30 to $200 in ATF license fees for three years for most license types. The most common one, the Type 01 dealer FFL, costs $200 for the first three years and $90 for each three-year renewal, and the Type 07 manufacturer FFL costs $150 every three years. Destructive device FFLs (Types 09, 10, and 11) cost $3,000, and SOT registration for NFA items adds $500 per year ($1,000 for larger manufacturers and importers).
I'm a firearms attorney who has helped thousands of people get their FFL. Here's every cost you'll actually pay, starting with the ATF fees, then renewals, SOT, and the extras most people miss.