Currently, December is the cheapest month in which you can book a flight from Washington, D.C. Dulles Intl Airport to Carlsbad (average of $445). Flying from Washington, D.C. Dulles Intl Airport to Carlsbad in June is currently the most expensive (average of $798). There are several factors that can impact the price of a flight, so comparing airlines, departure airports and flight times can provide users with more options.