In Sanskrit, donation is referred to as daksina, spiritual giving for the teaching received.
In the West we are not used to this concept of daksina. In India it is part of the culture and tradition to honor the teaching and the teacher for sharing the knowledge and practices.
Really, we cannot put a price for knowledge or practices that help our hearts to relax.
We can give as much as we can whenever we can to express our gratitude, according to our situation and means available.
We can ask ourselves ‘How much do I receive for my offerings/work?’ This can be a good starting point to determine what feels appropriate to give.
For more information on daksina, see Resources