If you base it on a 5 day week it amounts to Baht 12,000 a month (assuming 4 weeks in the month). If the work is 7 days a week then it equates to Baht 16,400 for a 4 week month.
I cant believe any of these companies have such small profit margins that they cant afford to pay these amounts of money. It is just greed on behalf of the employer that stops them paying a decent wage.