I understand the concept, but the math doesn't add up. There's not
nearly enough additional water in the cheapest latex to account for the
huge impact on the price.

In both of your examples, assuming that the ingredients are indeed
listed in decreasing order by percentage, the better paint has more
binder than pigment, and vise versa for the cheaper. If the amount
of pigment needed to turn a given amount of paint white (the color of
titanium dioxide) is roughly the same, then the difference is the
amount of binder--and from my own experience, this can make a
huge difference in the quality of the paintjob and how long the paint
lasts before you find yourself scraping and painting again. It's worth

it--go for the better paint.
--Glenn Lyford