That is line by line for one person, from people actually living here, not an average scraped off a website.
Look at where the gap comes from. Rent runs 10,000,000 walking in and asking, against 15,000,000 through an agent. Electricity is 500,000 to 2,000,000 depending almost entirely on the air conditioning. Food is 1,500,000 if you cook from the market and 4,000,000 if you eat out. Internet is 190,000 to 310,000, a phone is 150,000, and a motorbike is 1,000,000 to 2,000,000 a month.
Nearly every row roughly doubles. So Da Nang is not expensive or cheap, it is both, and which one you get is decided by how you do things.
Not in those totals, because they depend entirely on you: visas, insurance, gym, laundry, travel and going out. The visa one is sneaky. Renewing every ninety days is about 220,000 VND a month if you do it yourself, closer to 1,300,000 if an agency runs it.