Anyone who has ever worked for a nonprofit will probably tell you that most people working in the field are underpaid. However, compare their salaries to what managers of for-profit corporations of equal size, with similar responsibilities, make. However, encouraging slightly lower than private market salaries is a good tradition for the nonprofit sector. Lower salaries insure that the people who fill these positions are there because they care about the organization's mission, and not just for the money. Nonprofits perform services which are difficult for donors to evaluate, so the people responsible for delivering these goods and services must be especially trustworthy.
Lower salaries is one way to screen out individuals with less than noble motives.