breasts are more expensive because of demand and supply. There are less white meat than dark meat on a chicken and in general, people buy more breast than dark meat (cause in general you're paying for all meat compared to bones for dark meat). I know there's an article somewhere but most home consumers buys breast and it's supply and demand when i learnt it back in the days in school.
found it.
http://wiki.answers.com/Q/Why_is_whi...k_chicken_meat