Actually if you do not include subsidies by developed countries to their farmers (a big if) farmers in India China or Vietnam, could earn the same as a farmer in the US or Australia, there is a market price set for agricultural products, which they could charge which is one and the same for all. The fact that they don't is due to largely structural issues with those economies. Agriculture is a very warped industry But I am digressing and being pedantic.
The question asked what was fair to pay someone to be a domestic helper not what the market rate was. fairness is a very ambiguous concept and varies from person to person. Personally I see it the way I see it. If you ask me what is fair, then that would be paying someone the same amount that I would be willing to do the same work for. The fact that there are people willing and able to work for far less is the way the market works, and prices are set accordingly. I don't think that the market prices are the fairest and most equitable way of setting prices, but it is the most efficient and assuming there are reasonable amounts of competition, consumers benefit from lower prices.
10 years ago in Europe, hardly anyone other than the rich would have had domestic helpers and that is probably still largely the case, even with an expanded EU today (at least this is the case in the UK still). That is because the market rate is so high because very few citizens were prepared to do such work or not at an affordable cost to the middle class. What do we do in South East Asia when faced with this problem, we import as much foreign labour as we desire from impoverished countries, give them little or no rights, no recourse if they have monsters as employers, and pay them less than US$ 500 a month, which is less then the going rate for unskilled labour in other industries. The fact that quite a few of these helpers have tertiary education as well and cant get work visas as skilled labour is lost.
I am not disputing the fact that being able to work in Hong Kong, gives these people or their families back home anyway, a better standard of living then they would otherwise get. Does the whole concept seem fair to me. No it doesn't sound very fair to me really. Do I have a maid, yes I do, and she gets paid more than the minimum a month and only does part time, but no where near the wage level that I would need to be paid to do the same work, but I have never argued that life was fair either.