How about a set of measurements for the stock Apple earbuds, or the cheapies that come with a player from the drugstore?
A graph of, "This is the theoretical best" (a straight line), a $1000 headphone (HD800, maybe), a budget headphone, and the free/cheap/junky ones might be a very visual way to point out how horrid the cheapies are.
It might also not be visible enough to show the differences.
