I haven't been to GC, but I know that St. Barth's is VERY expensive. Do some research on the web in regards to hotel prices. That should give you an idea.

The biggest cost is usually dining out, so if you can find a room with a kitchenette, that would save you some bucks.

topping.

We've been to both cayman once and St Barth 9 times. we found Cayman way overpriced. The food is expensive on St Barths but the quality excellent. Cayman is outrageous even for fast food. There is also no comparison in the beauty of the islands Cayman's Beauty is all underwater and St Barths all above the waterline.

We'll be in St. Bart's next week, so will have better info then. Have never been to Grand Cayman, but spent a week in St. Bart's many years ago and since then have been to Anguilla (2x) and Tortola. Here's how I would look at overall expense:

We rent a villa, which makes the weekly lodging pretty reasonable. For a good meal out, expect to pay big city dining prices -- that goes for all islands, in my opinion. The difference on St Bart's is that the quality is extremely high. If you balance fine dining with local specialties you should do fine.

We also found that for grocery supplies and gourmet level take-out food, St. Bart's was very reasonable. Fabulous bread (best outside Paris) and delicious croissants are very inexpensive to have with coffee on the deck of your villa makes a lovely breakfast. Liquor and wine is pretty reasonable, so have before dinner drinks on the terrace or walking along the beach and save on the liquor tab when you go out....you get the idea.

Let's face it, it's a real luxury to travel, and on the Islands, where everything has to be flown/ferried/shipped in, you pay a premium. But I definitely think it's possible to have a fantastic vacation and not raid the bank.