From June 1st onwards, international transit/transfer (stay in the transit area) passengers must have a health insurance with minimum coverage of 10,000 USD or as prescribed by the government which covers treatment throughout the entire duration of stay in Thailand.
(The insurance policy is not applied to Thai nationals). You can check cost on this link
They can offer you the cheapest health insurance companies for you. They are agent insurance from Aetna , LMG, Pacific Cross , Aetna Them insurance License : 5904035648