We have been to Maui twice. once just DH and I, the next time with 2 of our children (age 20 and 16 at the time). Both times we stayed in a condo with a full kitchen. We never really "cooked", just went to the safeway and stocked up on soda, bottled water, sandwich fixings and snacks. I was so scared about food and gas prices! I had heard all kinds of rumours about the outrageous costs of these things online! Well, it wasn't nearly as expensive as I had read about. It was higher than at home (Oklahoma), but our state, and our area, has a very low cost of living (average home is between 100 and 150 K). I wasn't any more shocked by Hawaii prices than I am Florida/Disney prices. Don't stress it, you will find many cheap places to eat, and be sure to ask the locals where they eat out... that will be the best way to find the cheaper, and better food!
One bit of advice... be sure to invest in the book "Oahu Revealed". We used the Maui version, and it was SOOO helpful! It made our vacation 100 times better! It gives you all the info about sites to see, hotels, food, renal cars, just everything! And it's written like "at mile marker 14, stop on the side of the road, hike 1/2 mile to the left, and find a waterfall that few tourists ever see"----stuff like that! It is WONDERFUL!!!!
Have a great trip! You have made me want to go back!!