I'm looking for the cheapest round-trip ticket from Ho Chi Minh ( Vietnam ) to Los Angeles. The minimum price is around 670 USD. On the contrary, If I book the ticket from Los Angeles to Ho Chi Minh (Vietnam), it only costs 493 USD.
I used Skyscanner / Momodo / Google flight, etc. to try to get the better deal, but those do not work.
Do you know why the flight from US cities to Vietnam is cheaper than from Vietnam to a US city?
Flights are priced based on market, not particularly distance flown. So frequently prices going one way can be different than the reverse route. Likewise the number of fare classes (fare buckets) can be different, so the jump between fares can be greater.
Though, I find the opposite of you for flights I deal with (Thailand-USA and China-USA) as round trips fares from Asia side seem to be lower than from the USA side.
As a general rule of thumb with airfares, forget any logic, it is purely market driven pricing.
As someone already suggested that's the market. I would advise to check flights from Bangkok, KL or Singapore to USA and find if those are cheaper and then book cheap flight from VN to those destinations.
