Answer by Professional Japanese Teacher
Are you serious?
There are several ways of saying “Really?” in Japanese. The first three are casual expressions to use if you are on close terms with your friend.
If your friend is just an acquaintance, then you can use the last expression which is more polite. However, having said that, 100 million yen is a lot of money so it’s perfectly normal to be surprised at the news.
Thus, it is also acceptable to use a casual form when you’re surprised.