Good article from Cindy Sadler writing for Classical Singer:

Know when to say no..If they are paying everybody but the singer, they don’t deserve professional quality work. They should be getting a student or an amateur for whom the event could be important and worthwhile. They should be getting what they pay for.

This is one of those difficult issues for young singers, and for every person it’s different. The sooner you ask yourself what you feel you’re worth, however, the better. Listen to your feelings before and after jobs and ask yourself if you felt appropriately compensated for your time, effort and expenses.

Then stick up for yourself. Very few will do it for you.