I think Johnicholas is assuming a model where your pay comes from an organization that would otherwise use the money to help the world in other ways, and you're "underpaid" if and only if the work you are paid to do is more helpful than the alternative uses of the money would have been.

Of course, if you're being paid by an organization that would not otherwise use the money well, it's extremely easy to be "underpaid" in this sense.