How much do you tip an Everest Sherpa?

I'm curious, when it comes to compensating an Everest Sherpa for their invaluable guidance and support during a treacherous ascent, how much is considered an appropriate tip? Do factors such as the duration of the climb, the level of difficulty, or the Sherpa's experience and reputation play a role in determining the amount? And is there a standard industry practice or is it more of a personal, subjective decision?