Taking out the bigger cities would not make any difference. Government school salaries are nationwide.
Most teachers will retire at a similar level so long as the length of service is the same and they gave passed "Ajarn 3". That will be 40k+ baht a month. When you add in their free healthcare, for them and family, cheap loans and a very good pension scheme, they do very well.
Also add that they have more flexibility to arrive late, leave early, take long lunch breaks and get paid days off. They are better off than foreign teachers by a long way.