CK, I would say, "a lot".
Danny, as I understand it, a fresh med grad will first have to work as an MO in government clinics, and work on specialising along the way. The pay as MO or GP is not really fascinating, and after a few years may be comparable to that of a GP in private practice.
See:
http://www.salary.sg/2007/income-of-gp-doctors/
For a specialist, the pay is of course significantly higher.
See:
http://www.salary.sg/2007/doctors-pay/