] I just watched P&P2 last week and something bothers me. When Mr. bennet finds out that Lydia has run away he say's to lizzy that he doesn't have enough money to buy Wickam off because he never saved. He intended to father a son. But how does that relate to the matter of money? Even if he had a son the amount of money he has stays the same or doesn't it??

Yes, his logic *is* rather weak. I think what he means is that

a) if they had had a son, Mrs. Bennet would not automatically lose her home on her husband's death, and

b) Mr. Bennet expected that his son would help him break the entail (not a foregone conclusion, IMO), thereby freeing up more money to leave to Mrs. Bennet and the girls.