An American told me that 30 years ago, the banks will give a mortgage to the borrower to buy a house for not more 3 years of the income of the breadwinner (and the income of the working spouse is disregarded). The bank also required the borrower to make a down-payment of 10% for the house.
If this rule is applied in Singapore today, the average price of a 4 room HDB flat should be $120,000 (assuming an average income of the breadwinner to be $40,000).
The price of HDB flat that is being charged is about 3 times of this benchmark - and a 4 room HDB flat is considered to be quite modest, compared to a typical house with a land in America. (See http://en.wikipedia.org/wiki/Public_housing_in_Singapore)
This shows how costly are HDB flats in Singapore - and they are considered as "public housing". No wonder, both parents have to work and they still do not have enough money for retirement. The housing is far too expensive.