Is it possible to spend only US $1 (30 thb) in a local rural Isaan market for dinner?
Yes, it is!
Last night I bought the following:
(2) Moo Ping 10 thb, (1) Bag of sticky rice 5 thb, (2) Banana Nut Muffins 10 thb and (1) Small bottle of water 5 thb.
Grand total: US $1 or 30 Thai baht
There are other prepared meals, cooked by the vendors and placed in plastic bags that you can take home and warm up and eat. Almost any rural Thai dishes you can think of.
If you were limited to $1 a meal, you could do it if you had to.
However, more realistically would be 100 baht, which would include a large Coke with ice and more food.