I think you need to add more context to get a totally correct answer. In your dreams - an idiom which means that whatever you are talking about is not possible (and can only be accomplished while you are asleep and dreaming where anything is possible) Richard says to Mary "I am going to work so hard that one day I will have a billion dollars!" Mary replies "Sure Richard, you will find that money in your dreams!". In other words, Mary doesn't think it is remotely possible for Richard to one day have a billion dollars.