Hi there. I'm actually a grad student at UH now. The main campus is located in Honolulu, just off H1 and in the entrance to the Manoa Valley.
For starters, I recommend visiting. I VERY HIGHLY recommend visiting before making a final decision on grad school or an apartment. Many landlords don't take people seriously unless they visit the apartment in person. And quite honestly, the school isn't exactly a beauty to behold.
You can probably find a good studio or one bedroom apartment for less than $1000/month. Don't expect to find anything less than $600, unless you wanna live with 7 other people in a 4 bedroom house. And if an apartment is cheaper, it usually means there's something wrong with it. It could be an unsafe neighborhood, or no parking, or as I said, sharing with a ton of other people.
The closer you get to campus, the more expensive and smaller everything gets. Landlords know they can get away with it, because students want to be closer to campus. Don't bring a car unless you absolutely have to. Walking/riding bike/public transportation is all very good around here. Gas prices suck and traffic is horrible.
Gas right now is $3.09/gallon. Expect to pay twice as much on groceries as you are used to paying. Milk is $6.79/gallon at my local grocery store. If I can find a 12-pack of Diet Pepsi for less than $5, I pick it up. Hawaii has taught me to become less picky of an eater, and just buy what's cheapest.
Ordering a pizza will cost no less than $20. Most restaurants (especially in Waikiki) charge an arm and a leg. But surprisingly, the alcohol is cheaper in grocery stores and bars that aren't in Waikiki. So Hawaii has also taught me to be a little bit of an alcoholic too.
It can be quite a change to move here, and not just in the cost of living. If you have any more questions, I would be so glad to answer anything. I didn't have anyone tell me what it was going to be like here before I moved here, and I really wish I had. So please feel free to ask anything!