In general, at research universities, it's physics which subsidizes humanities. The university takes about half of every grant as overhead - i.e., science pays for the rest of the university.

As for my thesis that she's paid poorly because she is having fun, that's the situation for all academics (including myself). If she is truly underpaid relative to the fun of her job, why doesn't she quit and find higher paying work elsewhere? In fact, why do so many people apply for such jobs, when so few actually are accepted?

Hint: she likes her 3 months summer vacation, flexible work hours, relaxed environment, complete freedom to work on whatever she likes, and the possibility of never-get-fired-ever job security.