Right-size a Lambda function

NimbusCart's order-processing Lambda was given its memory by guesswork, and the bill is higher than it should be. Use the right-sizing simulator to drag the memory slider from 128 MB to 1024 MB. More memory gives the function more CPU, so duration drops then plateaus, while billed cost (GB-ms = memory x duration) follows a U-shaped curve. Read the simulator to find the cheapest setting.

Question: As you drag the slider, duration drops sharply at first and then barely improves above the middle of the range. Reading the cost curve in the simulator, what's the best memory setting for this function?