June 2010 report on wages, MOM. Under the category, University and Poly Lecturers.

Mean: $7,170
First Percentile: $5,050 (from other threads, that's the starting pay of poly lecturers)
Third percentile: $8,750
Median: $7200

My guess is for a assistant professor, it runs between $6k - $8k.
Full fledged: $8k - $12k

Nothing conclusive, just a rough estimation.

I think my estimations are somewhere near, best that i can do with statistics and my understanding of rough wages. Don't forget they stay in campus and many rent their existing property for rents between 2k - 5k (HDB). Add that sum in and that's a 'fair' salary.

Fair, considering the time and money set for PHD education. 'Fair' considering how much your peers has earned in corporate world.

Hope i helped. Feel free to correct me if anything's wrong.