Short answer: Take advantage of the offer and reques the upgrade.

Worst case that can happen is that you don't get the upgrade when checking in.

It's impossible to say how the hotel calculates what to charge for a standby upgrade, but if, for instance, the hotel uses the average price difference between a Garden View suite and a Riviera suite (and not the rates for your particular night - which seem very inflated), a standby upgrade in the 150-200Euro range actually woul make some sense.