Jim and Della do not have all that much money. That is partly because Jim is not making as much money at his job as he used to. You can find this in the fifth paragraph of the story. It tells us that Jim used to make $30 per week but now he is only making $20 each week. We have no idea why that would be.
As far as I know, Della did not have a job outside the home. There is no reference in the story to her going to work or anything like that. The story also says "Twenty dollars a week doesn't go far." That implies they have no income but Jim's.