"Yes, yes, they must marry. There is nothing else to be done. But there are two things that I want very much to know: one is, how much money your uncle has laid down, to bring it about; and the other, how I am ever to pay him."

Mr. Bennet sums up the situation with Lydia. Marriage saves her reputation, it is the only thing that will save her from a life of ill-repute. Mr B knows that Wickham must be "bought" for his daughter and the money cannot come from him.