Apps such as Agoda, eDreams, Trip.com, and MyTrip show that Bangkok-Chiang Mai fares between June 17 and 20 are VND1.3-1.4 million (US$51-55).
In Vietnam, fares between Hanoi and Da Nang on the same dates cost VND2.5-2.7 million.
Both flights take 75-80 minutes.
A Hanoi-Khanh Hoa flight in mid-June costs VND4 million, including taxes and fees of VND1.4 million.
The fare between Bangkok and popular Thai tourist destination Phuket is only VND1.5 million, including VND300,000 in taxes and fees.
Industry insiders say taxes and aviation fees in Thailand are only a quarter of Vietnam's, and that is why domestic airfares in the former country are "budget-friendly."
"Reasonable prices," Tran Huy Cong, head of sales development at Vietravel Airlines, said when referring to Thai domestic airfares.
Cong himself once booked a round trip ticket between Bangkok and Chiang Mai for only VND1.5 million.
One of major reasons for the cheap fares in Thailand is that the aviation industry is very supportive of airlines, he said.
Thailand currently has 12 airlines, including six major ones, Cong said.
Except Thai Airways, the remaining five major ones have equal potential and compete "very fiercely" on price, he said.
"These airlines have to continuously reduce costs and offer cheap prices to attract customers."
In Vietnam there are only three to four operating airlines, of which Vietnam Airlines and Vietjet Air account for 90% of the market share, resulting in a lack of competition like in Thailand, Cong said.
Thai carriers did well last year, carrying 64 million domestic passengers, 1.6 times Vietnam's figure.
Thailand also gets 2.5 times the number of foreign visitors that Vietnam does.
Last year, the Airlines Association of Thailand called on the government to cut fuel tax and operational fees at airports amid concerns over a potential increase in oil prices.
Major airports lowered charges imposed on airlines in an effort to boost flight frequencies.
Thailand identifies tourism as a key industry, and so the government looks to support airlines and tourism businesses.