The device itself has no monthly fee.
There are costs if you want to use things that cost money. (Netflix, Prime Video, Hulu, HBO etc.) There are also things with commercials that typically (but not always) cost nothing. There are also channels that can be logged in for no additional cost if you already have another TV provider like cable, satellite, hulu etc. There are a few channels where I don't even see a "business model" and perhaps they're just doing it for fun or are in a startup mode (attempt to make money later.)
There are ~ 5000 channel/apps available on a Roku device so there is no single answer.
It's probably best to think through the options and don't sign up for anything when aggravated.
If you want to poke around without getting into too much trouble, you can go in this way:
https://my.roku.com/signup/nocc
You can also add or delete a payment method as desired later on.
