Like Rob said, it all depends what comfort means to you.
If public transportation and living in the suburbs (in a small
condo appartment) is comfortable for you, as is eating in the local places or cook your meals at home (already more expensive than the hawkercentres), I reckon you could be ok with around 5K per month.
If you'd want to stay downtown, follow the action and do your clubbing, outings, eat in the restaurants and love alcohol, and you feel the need for driving a car around, you would have to triple that amount at least I guess. So around 15K per month.
All in Singapore dollars of course.
Eric