@tim, i'm sleepy so will get back to this tomorrow, but i've lived seven years (except this year) on oahu, in honolulu. i've also been to the big island and maui. i'll be back in honolulu come april. i can answer some of your questions.
briefly: yes, differences in both things on different islands
milk: $4-6/gal, and not because of shipping (prices were set to "protect hawaii's dairy farms" but they closed them all as i was leaving).
welfare? same as cali, that one guy's mistaken (or his link is).
anyway, gonna hit the hay now. you can also pm me about it. i love hawaii and would enjoy answering questions.