Actually the mean wealth is positive: which means that the current value of HDBs actually offset the debts in which Singaporeans are: home, cars, taxes. On the other hand 30k represent a small variation of the real-estate prices, it could easily go up or down.
For the record, Singapore’s median wealth per adult actually stands at just US$30,092. There is a huge difference from the average wealth which suggests that a lot of wealth is concentrated at the top echelons.
If we consider the fact that most Singaporeans would have a sizable amount of their savings in their CPF, the median disposal wealth per adult would be much lower. Actually, even the entire sum of US$30,092 will not be enough to buy a 3-room HDB apartment.