Da Nang doesn't have the flight volume of Bangkok or Singapore, and that scarcity shows up directly in nonstop pricing from North America and Europe — a direct routing is often the most expensive way to get there, not the cheapest.
Routing through a regional hub — Bangkok, Singapore, Seoul, or Taipei, depending on where you're flying from — regularly opens up meaningfully lower fares than a "direct" search shows, even after accounting for the layover. It's a pattern that holds across a lot of secondary Southeast Asian cities, not just Da Nang: the big hub gets the price competition, the smaller onward leg gets tacked on relatively cheaply.
Why this works
Airlines price the trunk route (your home city to the regional hub) competitively because multiple carriers fight over it. The connecting leg to a smaller city like Da Nang has far less competition, so pricing it as a separate "hub-and-spoke" itinerary rather than searching for a single direct fare often beats the all-in-one search — sometimes by a wide margin, sometimes only modestly, depending on the week.
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The trade-off is obvious: a longer total travel time for a lower price. For a multi-week trip, saving a meaningful amount on the flight in exchange for a few extra hours in transit is usually worth it. For a short trip, it may not be — run the math on your specific dates before assuming the connection is the better deal.