We have no clue about what the eventual usage situation within PrPro will be with frame.io. I'm not sure anyone even within Adobe has that yet. I've talked with up to the head of the Adobe video apps, and he is still scrambling to keep up with internal discussions. I don't think any decisions have been made yet.
And it might be several months until we get a rollout, and more until there's a 'final' set process.
That said, personally I doubt from what I've read, seen, and heard that there will be additional monthly fees for "full" CC subscribers.They're pushing collaborative workflows and wanting as much benefit as possible to full CC users that I highly doubt they'll have a fee for full-subscription CC users.
Remember, the Team Projects option once involved either an Enterprise license or an additional fee. That got dropped, and I'm pretty confident ain't coming back. I work regularly with some on the TP teams, and there's no discussion there. They are trying to get that working as smoothly as possible for all users.
But quite possibly there could be an additional fee for frame.io use for "single app" subscribers. And if I'm doing full-on blue-sky guessing here, I'll posit that single-app and non-CC subscription users will pay a similar fee for the paid part of frame.io, and the current free part will stay as-is.