If you only compare DIVE costs you are correct. But what about food and lodging?
We typically do a week of boat/shore dives and GC was a lot more expensive for us although IIRC our flight was surprisingly reasonable - maybe even less than connections thru Oahu. Up until a few years ago when they merged with AA - we could get good deals on America West/USAir as they competed non-stop out of Phoenix with Hawaiian and Aloha. But those flights are gone now.
When we go to Maui, food prices (even in the markets) are a big budget item but on GC we only did minimal shopping and our bill was easily over $200. And it was not enough food for the week if we'd wanted to do all our own meals - which we don't - it's vacation.
YMMV
Food is quite expensive on Cayman but car rentals, diving, and flights are usually not so bad. There is a wide range of prices for lodgings but if you stay on Seven Mile Beach you will probably pay the highest rates. We prefer to rent an apartment (car rental included) on the beach - not the powdery white sands of 7MB but there's good snorkeling right outside our door. We stay in a quieter, more residential area of the island, and the price is very reasonable, but we have to do some driving. That's not a problem because nowadays we mostly prefer to go shore diving on GC with occasional boat dives. If I were there for a dedicated dive vacation, I would look for more convenient options - like we do on the Sister Islands, and there are some reasonable all-inclusive packages available on Brac and Little Cayman.
You get what you pay for and if something is essential or important to you than you should include it in your budget. I am sure that there are many options and price ranges in Hawaii too, if you shop around. But I just don't care for blanket negative statements about a place, without qualifications, because it may not give a complete picture.