Money (more accurately, the lack of it) hangs over the story, forming the major pressure on the family. Gregor, once transformed, can no longer be his family's income source, which makes his transformation more difficult for his family to bear. He also blocks his family from making income from taking on boarders when he creeps up on the boarders during Grete's concert. Part of the story's humor comes from the fact that the lowest-class character, the charwoman, is the most at ease with the cockroach. In contrast with the boarders, the poor charwoman is equipped to deal with absurdity and grubbiness, and treats Gregor with humor and interest, if not real kindness.
Even as he becomes more and more cockroach-like, Gregor spends a huge amount of time worrying about his family's financial situation, demonstrating how fundamental money is. Even an insect can understand that abundance is better than scarcity. He does his best to eavesdrop on his family's conversations so that he can understand their finances, and his inability to do anything for them, more than his physical situation or the inconvenience he's caused his sister, contributes to his "shame and grief."
Money Quotes in The Metamorphosis
"What a quiet life our family has been leading," said Gregor to himself, and as he sat there motionless staring into the darkness he felt great pride in the fact that he had been able to provide such a life for his parents and sister in such a fine flat. But what if all the quiet, the comfort, the contentment were now to end in horror?
"I'm hungry enough," said Gregor sadly to himself, "but not for that kind of food. How these lodgers are stuffing themselves, and here I am dying of starvation!"