What I commit is time. And this can not be replicated, as in selling copies of an ebook. Since there are only so many hours available, the monthly fee needs to be adjusted so that we are both content. You with the level of support provided. Me with the financial return.
Nothing is cut and dried.
There are no signed contracts . Just a mutual understanding that can be adjusted as needed. You will need more support in some months than in others. But if the balance suits both our needs, differences month to month do not matter. If demands on my time continue to increase, the monthly rate increases.