I live in the Bay Area, so $20 per hour is standard for a background-checked nanny through an agency. Doesn't shock my conscience. Hawaii cost of living is pretty similar to the Bay Area (and a lot of the tourists to Hawaii are pretty well-heeled), so I've always expected rates to be comparable.
Went last year and used Malihini Keiki Care (
www.mkcare.com) for my then 22 month old, which is one of the agencies that a lot of the resorts on the Kona Coast recommends. We had a fantastic babysitter who had clearly nannied for some of the most demanding of Hawaii tourists (celebrities, etc.). Used her twice to go out to dinner - couldn't quite use the whole 4 hours, but it was worth it. I think if you have a younger child, it is worth the peace of mind to go through an agency that does background checks.