In "The Rocking Horse Winner," how does the boy's mistake about "filthy lucre" clarify the mother's thinking and her motivations?

This is a key moment in the text. We have already been
presented to the mother, who becomes obsessed with her supposed lack of money to the
point when even the house is filled with strange voices saying "There must be more
money! There must be more money!" The children, growing up in this environment, are
obviously impacted by this insatiable hunger for more money, which leads to the
conversation that Paul has with his mother:

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"Oh!" said Paul vaguely. "I thought when Uncle
Oscar said filthy lucker, it meant
money."

"Filthy lucre does mean
money," said the mother. "But it's lucre, not luck."

"Oh!"
said the boy. "Then what is luck, mother?"

"It's what
causes you to have money. If you're lucky you have money. That's why it is better to be
born lucky than rich. If you're rich, you may lose your money. But if you're lucy, you
will always get more
money."

It is important to
note that the word "lucky" plays a very importnat role in the story. The mother's
assertion that luck determines whether or not a person has money indicates her
single-minded focus on wealth and its acquisition and gives Paul the mistaken impression
that to be lucky is to be wealthy, whereas, obviously, there are lots of different ways
in which you can be lucky. It is this conversation that places Paul on the path to
self-destruction as he inspires himself to be ever luckier to gain more money for his
mother. The ending of the story clearly shows how destructive this view is, as the
pursuit of luck and money leads to Paul's death, ironically giving the mother what she
wants, but taking away one of her most precious possessions.