Short answer: Bangkok, Hanoi, Phnom Penh, Yangon — all $1 base cost. The cheapest serious tourism in the world.
Southeast Asia dominates the cheap-Asia list. Bangkok, Hanoi, Ho Chi Minh, Phnom Penh, Yangon, Colombo, Dhaka, Kathmandu — all $1 base. South India also cheap (Chennai, Kolkata, Bangalore at $1). See full Asian cities ranked.