Kudos for starting with financial boundaries. Maybe the frequency boundaries aren't ready to be set, per se, as you sort through just how much you are willing to withstand.
It's unfortunate, as far as being able to voice any maternal tips/thoughts/ideas that you are censored. It would be nicer to hear, you know I've considered that but chose this but I appreciate the input. But I'm guessing that isn't your daughter's approach. Maybe something to work through as you sort out the frequency of visits.
They live in a different state, or is my memory jumbled? That too can be a financial drain on them. Maybe tie that in as you think how to express your desire for less frequency?