Figure on 25 percent more than the US, in particular for meat. If you and the guys are intending on that hunk of prime beef on the grill it will be not only expensive, but somewhat disapointing; I've not seen a good rib eye at Bobby's in five trips.
Also figure on your lifestyles, if you intend to dinghy off to the local resturant for dinner and drinks etc. that too will cost 25% more than you expected to pay in the US. Some of the places you simply have to go to - Willy T's, Cane Garden (many good eats and drinks) the Sand Dollar, Foxy's, Leverick and Saba, etc.. Do stock up on the beverages (beer rum etc..) on the boat and do your serious buddy time drinking onboard - but do get out and see what the fuss is all about.
If you stay simple it won't cost too much. I've done 500 bucks for 8, resupplied some items (water, soda and ice) and spent another 3-400 in resturants and bars as an average. For beer, save money and open your pallet to the local beers. I hear Red Stipe is good (I don't drink beer so couldn't offer an opinion).
Do not count on fishing to supply your meat. Read understand and obey the rules down there regarding fishing.