Reading the nutritional information allows you to calculate how much of the product is water.
Reading the ingredients list can also be enlightening. From this morning's shop...
Sainsbury's own brand cream cheese is half the price of Philadelphia. Basics cream cheese is half the price of Sainsbury's own brand. However, added ingredients in the Basics version include wheat fibre and citrus fibre, presumably to enhance the texture and flavour. Presumably because the flavour and texture *need* enhancing. I went for the own brand in the end.
Interestingly, Philadelphia also contains stabilisers, which Sainsbury's own brand appears not to (no ingredients list, so I presume nothing added).
When I was a student in Leeds, I wrote a piece for the student newspaper comparing the price of a basket of typical student shopping in all the different Leeds supermarkets (and the Kirkgate market). There was a discount warehouse place called Giant where you could buy things like lard by the hundredweight. They had cans of tomato soup for something obscenely cheap like 6p. However, as I noted in my piece, the label of this tomato soup didn't count tomatoes (in any form) among the ingredients.
Does anyone else do what I do in supermarkets, namely walk round with a calculator in hand to work out the unit price of "special offers" and check whether they are actually the best prices? Often I find that "X% extra" or multibuy offers can be very misleading and aren't necessarily the bargains they appear...