If you are over 50 years old and your pension is at least 65,000 bht, you can apply for an OA visa, which allows you to stay in Thailand for 12 months with multiple entries, and when you get the last entry stamp, for example a week before the 12 months are up, you will still get a stamp with which you can stay in Thailand for 12 months without leaving from there.
In this case, the OA visa does not require a deposit in the bank, it is enough to prove with a legalized document that the amount of your pension is at least 65,000 bht.
The living costs depend on you, but 50,000 baht per month is enough when you include the rent of nice apartment.