There's quite a big gap between the going rate and working unpaid. I've worked for a couple of charities and most of the people there were happy to take slightly less wages than they could get elsewhere to do that job.
I've a friend who went into City finance for a big utilities firm after University. Pretty soon was on a six figure salary but then he left and joined a charity (big well known one, although he now works for another). He reckons he earns about 2/3 of what he'd earn if he stayed in the private sector which is still beyond most people's ideas,
the stories she tells of the inefficiencies and incompetence of the people working there and running it are mind-boggling. doubt many of them would last very long in the private sector. the amount of time and money that is wasted on simple tasks and operations is sad.
but there were such inefficiencies in the way they were investing their money that he saved multiples of his salary in no time at all by simply using standard private sector experience.
It's not for no reason that more and more charities are appointing people from industry to their boards and trustees.