The line doesn't have to get longer. The grader is incorrect under their own logic. Raspberries could be an unimaginable luxury, say on a moon colony or a very poor country unsuited to berry or fruit growth. Without knowing the value of raspberries, the value and cost of coffee is meaningless funny money.

Coffee keeps well and could be dirt cheap and it could be bad instant coffee that people don't really like. It may feasibly not be worth a single extra person standing in line for the bad coffee worth 0.75 zimbabwean dollars or 0.75 weimar marks.

This is pedantic and stuffy answer, but that is also what the "correct" answer is.