There is no need to make this more complicated than it really is. In this individual's case the conversation could go as follows:

> Q: Why do you want to work for us?

> A: Because I need the money.

Generalization is dangerous and people want to join groups of other people for many different reasons. Some of them are bored and have nothing better to do with their time. Others are charmed by that particular group of people because of who those people are or the kind of work that they do. A few are interested in moving to a different city or country and expanding their horizons. Making it sound like this is all about money everywhere all the time seems nihilistic and pretty depressing to me. I would not want to work with a person who is doing it just for the money or refuses to give a reason why they wish to join my group. I suppose therein lies the merit of asking "why".