I’ve been reading their FAQ and this sounds very reasonable:
If you want to use their cloud for backup, it will have a cost (they have not come with one yet because they have not come that far yet. It’ll depend on their hosting cost and such).
But if you want to just sync between your own devices without their server, you can, and it’ll be totally free (they insist that “your data is always yours and only yours”)
That's good to know. I'll keep an eye on it. I'm very content with Obsidian atm. For anytype the p2p device sync is definitely the alluring factor for me.