I'll second the advice of others: the cheapest way to fly inter-island is to use 5000 frequent flyer points to get a RT. At .015 cents per point, that's $75 RT (or $100 RT if you value your points at .02 cents per). If you are going to make 3 or more RT flights, then for 15,000 frequent flyer points you can get a pass for unlimited trips.

The second best deal is to visit the Bank of Hawaii ATM at the airport and get a Hawaiian Airlines flight coupon, charged to your Visa or Mastercard. You can make a reservation with Hawaiian Airlines right now and buy the flight coupons when you get to the airport. Each coupon is good for a one way flight anywhere in the islands. Currently, they are more expensive than usual, at $62 each. They can be as cheap as $52 each, so maybe the price will go down by the time you get here.

If you want to fly Aloha instead of Hawaiian Airlines, or you want to use a credit card other than Visa or Mastercard, then you can buy flight coupons from a travel agency that is within walking distance of the airport. Royal Adventure Travel (808-839-0039) is located right behind the Best Western Plaza Hotel (an airport hotel), so you can take the shuttle and walk to the back of the hotel. If you are too honest to use a hotel shuttle for which you are not a guest, don't worry. The place is a short five minute walk from the Diamond Head side of HNL.

Royal Adventure currently sells them for $62 cash, or somewhat more if you are using a credit card.

The most expensive way to fly inter-island is to just get the published fare of, currently, about $150 round trip which includes the fees. You can get that by calling the airlines or your travel agent.

BTW, Aloha Airlines currently has a published $94.50 (+ fees) round trip fare from HNL to the Big Island, but it seems to be an evil trick as all the dates appear to be sold out. Check ito it first, since if you are able to get it for the dates you are flying, it would be the cheapest way to go.