Go Beauty World Centre level 3 near the escalator, $6 haircut, always long queue, even by the high SES people living nearby.
I believe ground floor opened a competitor barber shop some time in the latter half of last year, also $6.
These cheap or 'express' barber shops don't shave your sideburns and the back of your ears and neck with a razor, if you look for that you might not be satisfied.
Alternatively, go to the famous Melayu Bugs Bunny barber shop in Toa Payoh. Should be $8 or $10.