@Strega summed it up nicely. The costs are solely dependent on what you want to watch. In my case, I pay for Amazon Prime for the shipping, so I consider the streaming as no cost. I get HBO Max via my cell phone plan. YouTube is free, although you have to tolerate a number of ads. You can rent stuff to watch from Vudu or Prime, so no monthly fee, just pay to watch.
Some services can be subscribed to directly through Roku, while some require you to go directly though the provider. If you want to access the content on something other than a Roku player, then you should not subscribe through Roku, as that requires using their devices. You have to be careful, as it's easily possible to end up paying more than you do for your cable service. Just be honest with yourself about what you really want to watch, and you'll likely find you only need a couple of subscriptions. And that should cost far less than your current cable bill.
To directly answer a couple of your questions. Billing can be monthly or annually, depending on the service. That does give you 24/7 access to their content. But also some providers might not stream content at the same time it's available via cable. Some popular TV shows might not offer to stream the latest episode until the next day, or you might need a more expensive subscription for immediate access. Just make sure you research the providers you're considering, and feel free to ask questions.
Dan
Roku Community Streaming Expert
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