Maybe you could ask people to fill out a little google form when they sent the receipt.Questions like “how many eaters are in your house” and “how often do you grocery shop” (or what % of your monthly grocery budget does this bill represent) might be interesting.

You could ask people to break down their bill by produce/meat/dairy/household/snacks sort of things. I think that would be too hard for you to figure out yourself.

What about questions like “what did you buy because of the price, on this trip?” and “what didn’t you buy because of price, on this trip?”