There is no monthly fee to actually use or watch the Roku device on your television.
However some Roku Channels do require a subscription fee to watch those channels which are pretty cheap per month.
But you can watch many other free channels on the Roku device without any subscription fee so all you have to pay is the cost of the Roku Device which is a one time fee.
And then if you want to watch any of those paid channels then you subscribe individually to those channels which are not owned by Roku.
I watch many other free channels on my Roku device without paying a monthly subscription fee.