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Domestic flights in Vietnam are not expensive, but not as cheap as the bus. Sometimes you have competitive offers, especially if you pre-book an airline ticket. Always check the booking conditions, in many cases the date is fixed and you will not get a refund if you want to cancel.
You can book the cheapest domestic flights via traveloka. This website is also 9 out of 10 times cheaper for other destinations within Asia than other websites.
To give a broad estimate, charge for a single ticket in Vietnam between $ 35 and $ 80. The prices depend very much on the season. Expect to pay more in the high season. Are you traveling during Tet New Year? Make sure you have booked your flights well in advance.