Flarum will offer, like other forum software such as Discourse and NodeBB, managed hosting by the creators. This will hopefully fund any cost and provide sufficient support for the software in the future.
I'm not a fan of having a managed extension platform with fees. Maybe a voluntary option to enable automatic deduction of revenue to donate, yes, but I don't think that a platform that doesn't make this optional would be used by professional's looking to make some money, if they can just distribute their software without such platform.