A single QP 201 card costs about $15 and is small enough to fit in a camera bag pocket. If you need accurate color, this is a worthwhile investment, but take care to protect the card when not in use. It is probably not necessary for the average photo hobbyist though, unless you have a specific need for accurate color. If flower or product photography is your thing, such a tool is worth having.
Argraph, the company that distributes the QPcard, provided a QP 202 two card pack for the David Busch Quick Snap Guide to Photo Gear.