It's actually not that low and almost double the pay that the maids get in HK. Young teachers only make in the low 20's in some of the schools, even less in language schools with no food or accommodation provided. The problem lies in getting a proper work visa which would protect your rights. Never mind the fact that it's illegal, that in itself is not a huge problem. The bigger concern would be to work for someone without any protection whatsoever, that's a lot of trust to put on people unless you know them very well.
Imagine they suddenly want you to work 15 hours/day with no days off or they hold your pay/passport. You get injured and have a big hospital bill. It's hard to go to the authorities when you're illegal.
The domestic workers in HK get sorely abused very often, not allowed to go out of the house except on days off. Even then, they have curfews, some families won't give days off and many of the maids go along because they don't want to get fired and be sent back to their home countries and employers know that and abuse them all the time. It's barely a notch above slavery...
Of course, there are nice families but I would be very, very concerned if I were you unless you know them very well.