OP - you may want to use flexible search as rates vary each day and then book multiple back-to-back reservations.

AAA rates: $246 for Fri and Sun nights, $255 for Mon, Thu, and Sat nights, Tue and Wed are $284-331. Regular rates vary in the same fashion, just higher ranging $269, $279 and up to $349. AAA rates at the Sydney Harbour are better than the rates at the Sydney Marriott which are going for $239 next week and $279 for most days the weeks after.

The least expensive rate I could find at the Sydney Harbour Marriott for this coming November (so far) is $331, and I check for lower rates every couple days. Last year the cheapest rate I could find was $253. So there are a few points of reference for you.

Any rate below $300 for Sydney Harbour Marriott is great IMO. Keep in mind that rates in Australia include taxes. Same goes for meals. When that steak costs $48 it includes taxes and gratuity. There is no tipping in Australia.

Australia is an expensive place to visit especially with the relatively weak US dollar. That will probably changing in the coming year or so, but that doesn't help you, or me, now.