I heard from my friends working at various ministries that the pay grade are almost the same. Which means:

2nd Upper (Hons) with NS: ~S$3.6k

Approximately $100-200 differences with every degree grade. Therefore,

2nd Lower (Hons) with NS: ~S$3.4k
3rd Class (Hons)/Pass with merit and NS: ~S$3.2k

For the ladies, you can expect roughly $200 lesser since you were not conscripted for NS.

The above are the basic salary. I believe the ministry may be willing to offer more according to your job experiences, the position you applied for (senior position) etc...