In Harper Lee's novel To Kill a Mockingbird, I believe Lee would agree that the poor are capable of greater sacrifice than others who are wealthy.
Those who are poor are often used to making sacrifices. Surviving is a difficult task and many times just keeping food on the table and shelter over one's family is hard enough. For the wealthy, they are not forced to sacrifice, and sacrifice is uncomfortable, so most people who don't have to sacrifice are not going to volunteer.
When Atticus agrees to defend Tom Robinson, the Idler's Club cannot understand why Atticus is planning to "defend" Tom Robinson. It is not that they cannot understand why Atticus says he will defend Tom, but why he is actually going to do it! This would be considered a sacrifice by some, but they cannot make sense of it.
The Missionary Tea women don't actually make sacrifices for the blacks in Africa, but what they do makes them feel like they are helping the "less fortunate." These women could do a great deal in their own community, but will not—not for poor whites, and certainly not for the blacks.
Those who have…are comfortable in this novel. Atticus does for others, but to him it is not a sacrifice. And in order to help people like Walter Cunningham save face, he will "barter" his services for whatever comes out of the Cunningham garden.
When Atticus finishes Tom's trial, the black community (people who do not have much) bring gifts of food as thanks. It is during the Depression and Atticus knows how difficult it is for them to feed their families. He is so overwhelmed by their generosity, that his eyes fill with tears and he tells Calpurnia to tell them not to shortchange themselves again that way. This is a sacrifice to these people, but they want to show their thanks. Even at the black church, though it "hurts," Rev. Sykes convinces everyone to give a little extra that week for Helen Robinson, and they do.