This is a classic WANT vs NEED. But to put a twist on it....If you WANT to save money, then you don't NEED expensive mineral water.
But seriously he doesn't need the mineral water it's what he wants.
If it is a health argument that the mineral water is better or better for you, then I would suggest a simple google search of 'truth about mineral water' and read up. Much less regualtion of bottled water than there is in public drinking water. A simple Brita filter or similar will save you money and give you clean water with no after taste.
jkf, i think you're missing the point. I believe it's been identified as a want already - they realize this, are still willing to buy it, but want to find the best deal they can. There's nothing wrong with purchasing/consuming something that is a want and certainly nothing wrong with trying to do it cheaper!