Whenever monastics charge fees (either set or “suggested”) for Dharma teaching, that should raise a red flag for sincere seekers. There also should not be repeated exhortations to the “generosity” of students. Monastics simply hold or place our begging bowls, and trust.
As instituted by the Buddha Shakkyamuni, the norm has always been: “If you uphold the Dharma, the Dharma upholds you ―dharmo rakshati rakshitah.” There are many arguments about the need to ‘adjust’ to modern circumstances, but they are unconvincing.
Even when we offer teachings and retreats at our own initiative (with expenses for accommodations, food, etc.), we must trust that participants will reciprocate voluntarily and spontaneously. If they do not, then it is an opportunity for the monastic to practice generosity.
If a monk or nun does not have sufficient confidence in this principle, then perhaps s/he should wait for an invitation from a sponsor, refrain from teaching the Dharma, or disrobe.
Householder Dharma teachers have no such restrictions, but we should be aware that we are entering a business transaction whenever fees are demanded or expected. Caveat emptor!