It is usually not the ones most well off that do the bulk of the contributions. I was account servant for 10+ years and knew who contributed and who did not. The bulk of the contributions came from 2 or 3 families. Everyone else contributed little to nothing.
Count me as someon who contributed very little. I would write a $10 check once or so every few years because the guilt was too much. It takes a lot of guilt to separate me from my money.