Don't you have compulsory labeling for price per kilogram (or the US equivalent)? Of course the effect is still there, but that's the price that generally matters.
Not that I know of. Many supermarkets label price per ounce, but I don't know if that's a legal requirement. On the packaging all that's needed is the net weight of the item.
You're probably not going to see it in like a local corner store, but those places you know you're paying a higher price already.