I just came back from a month and a half in Hawaii. I can answer some of your question.
The food-restaurants are pretty normal, lets say more like europe so its not THAT expencive.
I found it cheaper to eat outside than to cook food.
A flight to other islands is between50-100$, not to bad
The sights are at a normal price range, like everywhere in the states. The hotels are not cheap but not something that will kill your budget
Anyway Hawaii is an amazing place that is worth every dollar you spend.