It depends on a few factors, age, BMI, pre-existing conditions, what level of annual cover you would like, best to speak to a broker who can assist navigating this. Tenzing are very knowledgeable and helpful, you can request quotes here:
International insurers allow monthly, quarterly and semi annual payments as well as annual, I think most local Thai insurers are only annual. All inpatient policies are direct billing where no cash is needed to pay for treatment. Have a chat to Tenzing Pacific for some free quotes