I have never stayed/dived with Cobalt Coast, but we love the Caymans and have visited several times. When we are on Grand Cayman, where there are lots of restaurants and food choices, we rent an apartment or kitchenette so we can prepare some meals because eating out 3 times a day is very expensive.
When we stay on one of the smaller out islands of Cayman Brac or Little Cayman, we usually opt for an all-inclusive meal plan.
We like to find accommodations away from Georgetown and the seven-mile beach areas on GC because of traffic, heavy tourism, and all the cruise ship passengers. We always rent a car for the week on GC, but on CB we usually rent one just for a day to tour the island. You can rent scooters on tiny LC. We usually visit the Caymans for 2 weeks, spending one week on GC and one week on CB or LC or a liveaboard. Cobalt Coast is located away from the GC tourist sites so they probably won't take you to Devil's Grotto and other spots that have been beaten up by lots of divers - so that's good.
Check the currency rate for the Caymans, the US dollar is usually worth about 0.80 cents in the Caymans. I see that you are Canadian and I know that the Canadian dollar took a hit recently.
The Caymans are expensive but I think they are worth it and if you plan in advance you can reduce/control costs.
Why is it worth it? Anywhere in the Caymans you can eat the food and drink the water without concern about getting sick. The government takes mosquito abatement seriously so there is less risk of dengue, chikungunya, and malaria than you may encounter elsewhere in the Caribbean. The roads are well-maintained, the crime rate is low, and the health facilities are very good.
On GC there are lots of topside attractions and shopping if these activities are of interest. There is excellent diving available and many reputable dive operations. You can have the best of both worlds "vacation attractions" and "undeveloped nature that's off the beaten path". The water and the sand are gorgeous and seven mile is truly a "world class beach" so you should plan to visit it. The people are very friendly and nice, too.
I agree with your dive club, since the airfare is so expensive why not spend more time visiting the islands? A week is too short, you hardly have time to get over the jet lag and relax and enjoy yourself. Since your group is thinking of a 10-day trip why don't you consider chartering a liveaboard for a week? You could spend a few days on GC at a resort (within walking distance to 7MB and other attractions) and then get on a liveaboard for loads of great diving that includes all your food, drink, and accommodations. I don't know what the group charter rates are for the Cayman Aggressor but we've found it to be a fantastic liveaboard trip.