Did you try to negotiate? 50% for 3 extra hours is steep (assuming you get 4pm anyway as Amb). 9pm as camsean reports sounds more reasonable for an Amb/RA.

Will they let you sit in the lounge for free or do they expect you to pay for that, too? I remember hearing from a FT friend that they (IC SIN) have an "only one afternoon tea per paid night" policy. Reportedly this is to deter staycationers from booking a single night stay and availing of afternoon tea of the arrival and departure day, but this might pertain to your situation, as well.