How can you find out how much employees make at a non profit organization?

I work at a non-profit organization in Kansas as an office assistant. Starting out I made minimum wage, then I moved up to $7.50/hr for 2 years, and now, 3 years later, I make $10/hr. Most employees here stay at the $10/hr wage, but office administrators and department heads make up to $15/hr. I doubt the pay ever goes higher than that though. I have friends who work in other non-profit organizations, and their average pay is around $12/hr.