As far as I can tell, the minimum charge for an online table is DBU-cost x 24 x 30 per month, i.e. around $720 per month.

Is the 15-minute scale-down window documented somewhere?

With a 15-minute scale-down window, assuming just one access per hour, that should then result in a cost of DBU-cost x 1/4 x 24 x 30 per month, i.e. around $180 per month–is that correct?