Lawrence's main point about greed is that it is an ever-hungry demon that is never satisfied, and who asks too great of sacrifices.
The greed of the family is so powerful that it becomes an actual physical force in the house, whispering in paragraph 5 and throughout, "There must be more money!" like a ghost haunting the backs of their minds at all times. This also supports the fact that greed is never satisifed. It also indicates that greed overtakes and permeates everything. Later, (not sure which paragraph) even after Paul starts making big money, it states that "The house had been "whispering" worse that ever lately, and, even in spite of his luck, Paul could not bear up against it." Greed is starting to wear down on Paul, asking him to sacrifice too much. It has taken over his life. Then when he wins big and gives it to his mother for her birthday, it states that the whispers "simply trilled and screamed in a sort of ecstasy" instead of quieting down. Greed is ever-hungry; the more money it is fed, the more it wants. Then, at the end, greed asks a sacrifice that is not worth it at all: Paul's life. His last words refer to his quest for "luck", or money: "I am lucky!" Yes, he was lucky in money, but gave his life for it.