You know when you've got $20 to spend on groceries, and you need paper towels?
So you head over to that aisle and, of course, it's cheaper per-roll to to buy the huge bulk package. But you just don't have $17 to drop on paper towels this week. That $20 needs to buy food, too.
So you buy the 4-package roll, or if you're really strapped, the single roll.
That's a common - and surprisingly costly - cycle poor people get trapped in, according to researchers from the University of Michigan.