I basically grew up in Hawaii and have been to the outer islands around 100 times for sports. The figure of 5-7k is about dead on. Air travel like mentioned above is 1-2k. You should expect to pay 60-100 bucks a day for cheap rental car, 100-200 per night for a decent hotel. You can find older hotels across the street from the hilton in Waikiki for 75-90 a night. I would personally stay away from anything touristy especially luau's. To really see the island, you should find a rental car and explore everything. There are mind blowing hiking trails and natural beauty all over the place. The north shore during this time of the year is amazing because of the big surf and the place is the surfing hot spot for the world during this time of the year. Words cannot describe to see a wave with a 40-60 foot tall face.
If you need any recommendations for restaurants, just hit me up because I can point to to hole in the walls to high end places. My parents are there right now and I can easily shoot them a phone call to find out more.
Fyi, there is tons of night life to get yourself in all sorts of trouble into... especially the titty bars. Clubs and bars out there close a 4 am and there are a few underground after hour places that stay open till God knows when.