You'll find in actual reality, the "poor maid" is able to save most of that money each month, as all her other living costs are paid for by the employer, ie FREE electricity, FREE Air Conditioning, FREE GAS, FREE heating, FREE meals etc etc.
FDH's manage to save more each month, than most low paid/unskilled Hong Kong workers who serve you on a daily basis. Be that the door man of your apartment block, who earn between $5,700 and $6,800 per month, or the low paid waitress at KFC, or the dude serving you at 7 Eleven, who all have to pay their living,transport,food costs, with little to nothing saved each month.