Medicare does not like to cover the costs for these type of procedures. They will repeatedly tell you no, but if you have a doctor tell them that the surgery is the only option that will work for you, then they will cover it. Medicare will often tell you that there are cheaper ways to take care of apnea. Keep on them, they will eventually cave.
Being an elderly person who is 72 years old and on social security and needing something under $500 a month is very vague because you did not specify exactly what you needed. I'm going to assume that you are in need of a place to live or you need medications. Since you have not listed the city where you are from the best thing I can say is for you to do is to check your local county services for programs that assist seniors.