I'm sure that's not right at all. Even the medical care for foreigners isn't cheap here in Thailand, but the insurance is definitely not that high. Check AXA Thailand and get a quote... I'm sure they have a much better offer around 40-50k baht/year not stupid $900/month... Many people retire here at your age or even older and they don't even spend that much money on just living per month. Look around I'm sure you will find a better offer.
Sounds weird...$900/month (That's 31.600 baht) ... Can't be that much even if you are over 100 years old... (Except if you have already a lot of existing health problems...)
That's usually the yearly cost. Get help from a visa company and they can tell you the exact cost because this price is definitely not right.
If you are from one of the visa exempt countries (but depend on the country because few countries just get 15 days stamp on arrival) not necessarily. You get 30 days on arrival and for 1900 baht extending for another 30 days... Or come straight on a TR visa which gives you 60 days. The TR visa 1400 baht. So if the extra 500 baht matter TR visa is your choice if not just extend in the country. It's a maximum of a few hours in the immigration office.