for reference here are the current US federal income brackets. State income tax varies from 0 to 13%. A median US family earns 75,000/year and individual 30,000. The individual is earning 1 million THB per year. Not sure which rate they would pay in Thailand. My wife earns ~50k THB/month and pays 14% she says.
EDIT: there is also a standard deduction which is first applied to the total earnings.