Electromaster via Amazon offers the Snap-On Case with Card Storage for iPhone 4 / 4S in White (pictured) for $3.28 with free shipping. That's $1 below the lowest total price we could find for a similar case. This snap-on case features a pocket that can be used to store ID cards, credit cards, or cash.

Of note, it's also available in Black for $3.55 with free shipping. That's another price low by $1.