What is the theme in The Story of Philosophy by Will Durant?

Philosophers and finance is a theme in the book. Generally philosophers' thought may include materialism as an idea but rarely as riches. Even if they are from an aristocratic family and have inherited wealth, they end up poor. Another theme in the philosophers Durant selects for this work is their lack of financial success. Generally they are poor yet their great minds and ideas live in posterity. The rhetorical question remains open, if they are so smart why are they not rich?