You can find cheaper housing on the outer islands but they are much more rural. So just like living in a small town vs a big city on the mainland, you will have less selection of other things in life. like schools, doctors, shopping,etc. The big difference, on the mainland you can drive to the big city when you need to, in HI you will need to fly to get to Oahu. When I worked for WP we looked into living on the one of the outer islands. For us it was not worth the hassle of having to fly to Oahu for certain things. We were able to find a nice apartment for decent price after searching for a while. and that was on first year pay at WP (which at the time was much lower than HAL)
But I know several pilots that live on the outer islands, so just depends on what is important to you and your family. If you are single, it ill be 10 times easier.