How is the cost of living? Is it expensive?
This question comes up occasionally (not just Hawaii, of course). One place to start (but not too much on COL IIRC) is by our own Hawaii expert Nords:
https://www.early-retirement.org/forums/f47/faq-archive-e-komo-mai-hawaii-30671.html
If you can figure how to search, there are other threads - have fun if you're really interested.
vs. average USA mainland, I've seen figures like 65% higher COL. vs where I lived in the heartland, I'd guess that's about right (maybe a bit worse.) BUT (always a big BUT) vs Bay Area, maybe LA (the nice parts), NYC, Boston, and other BIG cities especially on the east coast, COL probably isn't much different.
As always, a lot of your COL is under your control. I always joke (well, it's not really a joke) I've not eaten a fresh blueberry on Oahu - only frozen. Back where I used to live, in season, blueberries would go as cheap as a dollar a pint box (in season). Here, it's more like a dollar an ounce for fresh. IF you don't mind riding a really good bus system on Oahu, you can save a TON by not owning a car (we own two, so stupid us.) You learn where to shop. Now that Sams, Costco, Home Depot, etc. etc., are almost everywhere in the Islands, you can save a lot.
Housing on, for instance the Big Island of Hawaii is much cheaper than on Oahu. Of course, we don't have numbered volcano zones here on Oahu, so there's that.
As mentioned by HI Bill, you might have to budget air travel to Oahu for specialists (and especially for emergencies - imagine an air ambulance from Big Island to Oahu. If you have to ask the cost, you can't afford it.
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We visited a dozen times and did a several week trial run to see if we could manage. It worked out well and we hope to stay put. BUT we do have a back up plan. By the way, prices of RE are just a bit soft right now due to Covid, so...