My wife and I have a house in a rural place in West Thailand. If we were given a monthly income of 40,000 Baht we would have no clue how to spend so much. We eat simply and healthily. Even with the (small) expenses of looking after 25 rescued cats, we spend very little because we grow a lot of our home food, and because local prices are low here.
But if you live in Pattaya, Bangkok, or another city in Thailand your expenses will be high because of rent, travel, food, beer, entertainment, health expenses resulting from city life, etc. Nevertheless, 40,000 Baht is easy to live on.
What about paying for health problems on that income? If you go to private hospitals, no. Government hospitals, yes. If you have been eating processed (supermarket) food for a long time, you will have health problems on the way resulting from your diet, if not already. Processed food is so harmful to the body. If you live simply and healthily, and you don't ruin your body with alcohol, your expenses can be very low. 40,000 Baht per month is a lot!