金 means “money”.
は is a topic particle which indicates that 金 is the topic he is talking about.
な is a particle which comes at the end of a word or phrase, usually used to convince someone in male speech.
命 means “life”.
より means “than”.
So 命より means “than life”.
重い means “heavy”. In this context, it means “important” or “worthy”.
んだ is used to make a sentence into an explanation for the previous context. So if it’s 金は命より重い without んだ he is just stating the fact that money is worth more than life regardless of the context. But with んだ, he makes it sound more persuasive in order to convince them to stop complaining about dying for money (He is explaining why they shouldn’t complain).