Anything of value is not free. Pure Dhamma is priceless - it is NOT free. Your cost to stay and learn has been paid by others that took the course before you. Similarly after you complete your course, out of compassion for others when you make your donation it will pay the cost for other students that follow you. Thus you don't develop ego - expecting services, quality for the "fees" you pay in advance. Generally people will not ask you directly to make a donation. There is no set amount. If you feel that you benefited from the course, then you donate per your capacity and goodwill.
Vipassana is completely free!
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