Probably RC awards and downtown hotels in the most expensive world cities will probably get you the best "bang" in terms of money savings. I suspect that trying to stay at one of the top-end Marriotts in downtown Paris, London, NYC, etc will most always be expensive though there may be some seasons where each may be a little less expensive than others. Also, there may by lower cost alternatives. For NYC, you could stay in a non-downtown hotel and drive/bus/train in everyday, or look at a non-high end alternatives.) But a top end downtown hotel during a high season, could cost you in $600 or more per night.

Another point of reference is that when I stayed at the Cayman RC, there regular rate for that same time period, if I had paid it, would have been more than $600 per night as well. It is possible that you could find at downtown expensive major city RC that is charging close to $1000/night, though you might not find it having Pointsaver award availability at the same time.