Bob Ewell lives in an old Negro cabin adjacent to the town dump, not far from where Maycomb's African Americans live in the Quarters. The roof of the house is made of crushed tin cans, and cheesecloth fabric serves as windows. The property is littered with junk claimed from the dump. Bob does not hold down a regular job, and he apparently exists off his monthly welfare check, though
... their relief check was far from enough to feed the family, and there was strong suspicion that Papa drank it up anyway--...
He is probably not the poorest man in town; many of the Negro families must be poorer. But Bob does not provide for his family, and his children go hungry and rarely bathe. Bob hangs out in the swamp for days at a time; his children go barefoot or make shoes out of old tires; the house has no running water, and the Ewells have to carry buckets of water from a spring by the dump; the kids play in the dump and are always sick; and Bob doesn't require his children to go to school. When Bob does manage to get a job with the federal Works Progress Administration, he loses it within a week
... and probably made himself unique in the annals of the nineteen-thirties: he was the only man I ever heard of who was fired from the WPA for laziness.
Well at the beginning, Bob Ewell comes to the Finches' home and pays Atticus with a bag of something (sorry can't remember what!!) instead of money. Scout asks Atticus why he doesn't just pay them with money, and Atticus tells her that it is because he is very poor and cannot afford to pay them with anything besides the contents of the bag.
If you mean how did he become poor, sorry, I do not know.