I lived in Hawaii until very recently, and some costs are more fixed than others ... honestly some of the best food is also the cheapest when eating out, but that's if you don't mind eating like a local: eg. plate lunches/bentos/noodle shops/etc. In fact, we often found it cheaper to do take-out than buy groceries and cook at home (though obviously cooking at home was healthier). There are a few higher-end experiences that are well worthwhile (the souffle at Roy's comes to mind, if you're staying at Aulani).
A lot of what the islands have to offer is incredibly cheap ... snorkeling at Haunama Bay has a small fee for non-residents (and buying your own snorkel equipment for $40/person at
Walmart or Costco is probably about as cost effective as renting it for a week, plus then you know you're the only one who has used it which is nice, though not very environmentally friendly), enjoying hikes in the various craters and rain forests and availing yourself of the beach are all free or have a nominal parking fee.
Some things are very worth paying for ... whale watching cruises if you're there between Nov-March are phenomenal experiences that never get old for me (and I still remember the one from when I was in 3rd grade ... equally awesome to the one I accompanied DS6 on his class trip). That can run you $40ish-$150 per person depending on the boat and where it goes.
Car rentals are a necessity in my book and will always be pricey.
Guess what I'm trying to say is that the average cost of a vacation in Hawaii can vary widely by what you choose. There are many that go to Hawaii, rent a bike with a trailer and haul around camping gear from beach to beach and stay for a month for a few hundred dollars. (I wouldn't personally, but that's not my style.) Others spend $4K/night or more on the best rooms.
One final tip. Airline pricing can vary widely depending on season, but despite the fact that their service tends to be ... well, not great, we often fly Alaska due to the price (and the fact that one ways and multi-city trips aren't penalized like most airlines do for anything other than a round trip fare). They have a credit card through Bank of America that gives you a $99 companion fare that works going to and from Hawaii, its the only companion fare in the industry that goes that route.