Google Now Launcher - The new Google Now will tell you when a price drop occur in your flight, if you are interested. This app from Google is continuously adding new features and abilities that are awesome, especially this new flight price tracking.
So, Google does this cleverly, if you are using Google Flights and through it when you book a flight, a helpful drop dialog will open on Google Now and it will show prices for flights.
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This feature has not yet, rolled out completely and will take time to be seen on different parts of the world.
Do, tell us if you have met with this new drop-down on Google Now.