Why is the cleaning industry willing to pay $3000 for foreign cleaners, but only $1400 for local workers?

inSing.com, 23 Sep 2011
It is difficult to attract local hires to fill cleaning positions in the cleaning industry even if the monthly salary is as high as $1,400 per month.
Employers have no alternative but to employ foreign workers at a much steeper labour cost of $3,000.
Although the monthly salary of foreign workers is lower than that of local ones, there are miscellaneous costs such as labour taxes, lodging and accommodation, medical insurance, and one-time round-trip airfare and agency fees to consider. Full story