There appears to be a norm of sorts, at least where first-time domestics are concerned. If they are Filipinos, they start at $320 per month and they get either one or two days off per month. For Indonesians, it is $240 per month, without any off days.
There is not overtime pay because it is not required by law. Domestic employment is not regulated by the Employment Act. The terms are negotiated between employer and the agents of the maids. The terms have remained relatively unchanged over the years and if they are changed it is to reflect a change in the situation.
Indonesian girls 10 years ago were paid even lower than the $240 are getting today. Then the Philippine government decided to give the Indonesian a boost. They banned the deployment of Filipino maids to Singapore. As a result of this brilliant move, the salary of Indonesian girls shot up to $230, then $240. Imagine the effect on the demand for Filipino domestic workers if the Indonesian maids were still get $160.
And when (1995-1996) Filipino maids were not coming to Singapore because of the perceived threat to their safety, those who brave enough to stay back in Singapore were in such great demand that employers would offer them generous raises to sign on for two more years. Sometimes the raises were not enough to fend off poachers.
So if you want to have a good maid working loyally for you for as long as you are in Singapore, set the norm yourself. I know two employers who did. Their maids worked for them for over ten years.
One of them got a ten-thousand dollar gratuity when she retired! I do not know what kind of gratuity the other got, but her employer has invited her over for Chinese New Year, all expenses paid, not to mention pocket money.
And very recently one maid I know off, will be going home for her second all-expenses paid vacation in less than two years. Her halfpat family (man is xpat, woman is ypat) go to New Zealand for Xmas and thought the maid might like to spend time with her own family. And, what the heck, 1k spend of airfare and incidentals is not such a big deal to them.
This maid is not going to give her employer any problem, you can bet on that. And when they relocate to Hong Kong in June, the maid will be only too happy to go with them.