I'm not a particularly healthy eater, but at the same time I really don't feel like filling up just before sitting down in a plane seat for hours. Consequently my purchases tend to be quite limited... coffee/juice/water, newspapers, maybe a light meal (McD's salad, sushi, that kind of thing). Oh, and the duty free when heading back to Canada.
I'm guessing that the amount is so low because many are in a rush and don't have time to spend a dime? I would wager that of the people who actually do spend money, the average must be substantially higher than $5.15. What can you get for less than that besides a convenience store-type drink or food item?