UN pays their volunteers reasonably well.  And you also get diplomatic status everywhere you go.  The perks are not all monetary in the beginning but eventually if you pursue a career in development it may also pay you much.  If you're into cultural diversity, travelling to remote places, charity, challenging yourself to contribute something to society or if you want to be a politician then that is a good start.  It's not all humanitarian work, you also learn how governments prioritize the use of their money and how they tie aid with certain interests.