There is no monthly fee for using an Amazon Fire TV Stick, Firestick 4K, Fire Cube, or even an old Fire TV device. When you purchase an Amazon Firestick or other FireOS device, you don't need to make monthly payments for subscriptions.
Amazon Prime Video, Netflix, Hulu, HBO, and many more have a ton of great content. All of these official apps charge a subscription fee to watch their streaming movies and TV shows. There are also "free" apps in the Amazon App Stre such as PlutoTV, IMDB TV, and even Crackle. Most of the official free apps make their money running as commercial television networks. If you don't mind watching a few commercials every few minutes, you'll love the content they provide.
By jailbreaking an Amazon Firestick, you also gain access to all the paid and free streaming apps available in the Google Play Store. You even get access to a huge number of free streaming apps developed especially for jailbroken Firestick and Android devices.