I've seen the "unit price" be per 100g (Especially so you don't notice it's a ton of money per kg) so there's still some screwing around...
If you had to convert from arms to legs or feet like in certain areas of the world, that might be much harder, of course.
Comparing Smarties to M&Ms is super easy if both have to use price per pound, price per oz, price per g or price per kg. Who cares if the price per kg would be $0.031.as long as I can see it's $0.028 for the other for the same unit without doing math.