Key Transit hubs in Europe generally are those which have the cheapest fares. From my own individual knowledge, I’ve discovered that London, Amsterdam, Frankfurt and Milan are typically cheaper to fly into than other smaller metropolitan areas. Flights is often filtered by multiple components for example airline, airport and cost; https://directoryecho.com/listings742007/details-fiction-and-how-to-find-cheap-flights-to-europe-from-the-us