Wow! 500 x $21 = $10,500 per year.
There is NO WAY this website (plus a couple free guns) costs that much.
Nice revenue stream.
So what? Someone saw a need, and created a business to fill it.
Also, as someone who has developed PHP websites, you probably know that as traffic scales up costs do increase substantially. Especially when you want the service to have any measure of reliability. A single dedicated server from a reputable hosting company will run into the hundreds of dollars a month, at minimum. If one were to separate the database and web server instances onto two separate servers for performance reasons, you can double that.
An endeavor like this would also require a nontrivial amount of upkeep (moderation, for example?) and socking some funds away in a "rainy day" legal fund is always a good idea. Perhaps doubly so considering the subject matter in this case.