In the UK the supermarkets (I assume they have to by law) quote the weight of contents of a single unit of product (which is supposed to be total weight - packaging weight), so you could analyse this quite rapidly.
They don't, some items are per unit, which is a way that avoid comparisons for fruit. In [my local] Tesco they even took away the manual scales now, making comparison across 5 different types of apples, say, much harder. Good mental-arithmetic practice.
