Costco, both in Kona and Oahu, has Foremost OJ from concentrate, a gallon for about 4.85. They also have Tropicana, and I think it was above $7, but even that's not bad for a gallon of not-from-concentrate juice.
Milk's always been expensive here. The good news is that gas is CHEAPER than in LA. 4.44 on Big Island last week, and I saw it for 4.26 on Oahu Saturday.
Plan on 20% more for groceries, and eat local food, not the boxed processed mainland food we all get used to, and you'll do well. In any case, if you compare it to restaurant meals, you'll feel positively thrifty.