Sometimes even Becky feels kind of guilty about her doings. When she does, she rationalizes:
"I think I could be a good woman if I had five thousand a year. I could dawdle about in the nursery and [...] and order half-a-crown's worth of soup for the poor. I shouldn't miss it much, out of five thousand a year [...] I could pay everybody, if I had but the money." The narrator is right there with her: "And who knows but Rebecca was right in her speculations--and that it was only a question of money and fortune which made the difference between her and an honest woman?" (41.38)
So what's the big deal about Becky's little fantasy? It points to one of the novel's big questions: do we do what we do because of our circumstances or because of our character? What do you think – could Becky be a good person if she had been born with a lot of money and didn't have to worry about supporting herself? What if she had suddenly inherited it as a young woman (say from Miss Crawley)? How would she be different? Or would she?