I tried to find a good discount rate for the Hilton properties in Singapore a couple of months ago, without any luck. None of the usual codes seem to work. I had to book regular advance purchase rate. I assume that the demand for rooms in SIN are so high that no significant discounts are given - hence also the very high rates.
When I searched for a room at a reasonable rate I came across the Holiday Inn Atrium, which had very reasonable advance purchase rates including breakfast. I didn't book it but on an earlier occasion I stayed at this property and was quite pleased with the hotel (although the location is not the best; you have to take the bus, taxi or a long walk to the nearest MRT station)