You're pricing the cost of apples vs. oranges, but yes, it differs by destination.
A nicer topside often entails a higher cost of living, such as with Grand Cayman. Do you like to get out & drive around the island? Do you like sandy beaches without sand fleas biting you? Would you like ready availability of modern health care? From what I understand, Grand Cayman offers all that. How's Roatan stack up? If you aren't at CocoView Resort, Reef House Resort of Fantasy Island Resort on Roatan, how readily can you shore dive? Does that matter to you?
I've seen pretty cheap round trip airfare to/from Grand Cayman. How's your ticket compare? Airfare to/from Bonaire is NOT cheap, although dive costs for shore diving are.
Cozumel and Roatan are known as over-all budget dive destinations (optionally; they don't have to be). A number of group organizers in my area choose Turquoise Bay Resort on Roatan, which was favorably reviewed in Undercurrent and offers a lot of diving for your buck.
Richard.