I guess Houston is just totally backwards from every other location when it comes to airfare. I can almost never get a flight to Coz for less than $600, but can usually find flights to GC for between $300-400. I do have to say that GC is just a layover destination for me. My experience has been when I factor in the quality of the diving, the service, the food, and the solitude of the location, the Cayman Sister Islands are the best bang for my buck. I hated the crowds and touristy flavor of Coz, and paying for everything piecemeal made it feel so much more expensive than paying one bill ahead of time and only worrying about incidentals and a tip after that. I understand that dining on GC can be quite expensive, but if you stay away from town in Coz, factor a cab fare into the price and include the time you wasted waiting on crowds, and you start realizing you aren't saving as much as you might think.
To me, especially on vacation, time is money. I only have a precious short time to relax and get rid of 30-40 weeks of stress, and anything that interrupts that (like taxi rides to town or fighting through pod people crowds to get to a restaurant), is burning that precious time like cash in a campfire. You might try to look farther out for flights or try some of the package deals on GC or the sister islands that include airfare. They usually have bulk airfare through Cayman air that is about $300 or so RT. Sorry to hear about your problems with the costs, but I do have to be a little selfish and say "more solitude for me" :10: (Just kidding!)