Hello, folks. I am mixing, effecting, equing, mastering, etc, etc a documentary feature that someone I know asked me to work on. Although I have mixed and mastered many projects before - songs, short and feature films, etc - this is actually one of the first times someone's actually offered to PAY me for it. They don't know what to pay me, and I don't know what to charge them. Given that I am willing to give this person the "bottom dollar student rate discount", I would like to assess how much that should actually be.

What is the standard rate that a "first timer" should charge for mixing and mastering a feature length documentary?

Thank you very much in advance, everyone, for the help and responses,
Javier Calderon