I have lived on Kauai for 2.5 years. It has been a great experience so far. Hawaii is a good fit for people who are FIRE. It's more difficult when you are still working. I teach college and get 4 months off/year, so it's not that hard for me. The people who tend to stay on the island long term are people who are FIRE or have a flexible job in which they can get off the island often.
1) Housing (Renting): For rentals, our rent is about 20% higher than market rate because we have a dog. Our first place was a studio at $1,100/month. Now we are in a 2 bed/1 bath for $1,500/month. Utilities were included for both rentals.
2) Utilities: Electricity is expensive. That's it. You can hack this system with a solar water heater, gas dryer and gas cooking stove. My old landlord would average 350 KWH/month for electricity and his bill was $100/month. This was for a multi-unit with 4 people and 2,000 sq. ft.
3) Air Conditioning: The only way to afford this is to have solar panels. Running a single window unit for one room at night to help you sleep will run you about $500/month during the summer.
4) Housing (Purchasing): I think the median single family home for the island is 600K. However, the island is very friendly permitting Additional Dwelling Units on your property. My wife and I are currently trying to purchase a single family home for 450K, put 50K into it for an ADU. With 20% down, our total mortgage payment would be $2400/month, but get $1500/month for a 2 bed/1 bath ADU. It's not cheap, but it's very doable.
5) Airfare: My wife and I fly from Kauai to Denver 3 times/year. We both have the Alaska Airlines credit card. Each card gives you a yearly companion fare of $99. We typically spend $750 for the first ticket and $150 for the 2nd ticket. It ends up being around $150 because of fees. This totals $1800 because we do it twice a year. Our third flight for the year is free with credit card points/miles. This averages to be $300 per round trip ticket. Not cheap, but doable.
6) Groceries: Safeway is about 50% higher than the mainland. However, Costco is only about 10% higher than the mainland. When you consider the year round growing season with farmers markets and neighbors giving you food, my food cost is the same as when I lived on the mainland. The only food I kind of gave up on was mushrooms. They are almost double the price. Now they are a treat when I am on the mainland. I actually ate them last night in Denver.
7) Beaches: free to park and you can bring your dog. About 90% of the people live within a 10 minute drive to the beach. You go to the beach often and it's awesome and free.
8) Amazon Prime: This has been a game changer for Kauai. Now you can get anything you want within 4 days. Sorry, we don't get 2 day delivery.
MMM did a write up about Oahu. I think it's pretty accurate. If you adapt to Hawaii life it's not that expensive. If you insist on keeping your oversized mainland lifestyle, it's going to be very expensive.