Customary $$ to donate after Vipassana retreat?

[PsiloPutty]

I have my first one coming up this month and wondering how much money to "donate" to the teacher-institution. I know that it's free, but I want to give. I was thinking $500, but is there an unspoken average amount that people tend to give? 

[PsiloPutty]

OK guys, thanks. I just didn't want to break protocol if there is one. Doesn't sound like there is!  

[Vladimir]

@PsiloPutty I'm doing Vipassana meditation too this month starting May 9th, are you going to the one in Twentynine Palms, CA by any chance?