you file a TM30 per permitted to stay stamp so it depends on how you entered the country, if on an extension of stay your original filing is good for the entire year stay, no need to repeatly file if returning to the same address.
You are looking at 150-200k baht per year. Existing preconditions excluded for a Thai health insurance. Prices vary greatly depending on age and existing symptoms.
it even depends on which IO you are asking, but nevertheless the Police Regulations were amended 5 years ago to remove the need for new TM30s when returning to the same address.
If returning to new address you need new TM30. If returning to same address at which you previously filed a TM30 no need. During your extension of stay process a new TM30 will be filed anyway.