What is the cheapest airfare from Taipei to Ho Chi Minh city? Can I get it from Taipei.

Once I took the plane and the journey costs about CNY1,500, excluding the fuel fare and airport construction fee. But the price is always changing, so I suggest you to look on https://www.travelchinaguide.com/china-flights/
And it need to stopover along the journey.
However, genenerally the air ticket costs around CNY4,000 for the direct plane.