From my experience and point of view, 25-30.000 THB per month is quite enough if you live alone and have a place to live. Single accommodation can be easily found for 5-6000 THB per month. If you have a Thai spouse 40.000 per month is OK. In rural areas, the cost of living is 20-25.000 THB per month. Of course, this is without a car and very costly health insurance which is a luxury and not necessary.