Most people rarely, if ever, go to
marinas since there are so many good places to
anchor. Our
experience is only with anchoring so I can't comment on how much staying in a marina would add. To some extent it depends on where you are since
food costs vary so much from one island to another. The things you have to spend on are:
food,
water (unless you have a
water maker and then you are likely to have increased
fuel costs, and adminstrivia (customs
fees, cruising permits, etc). The latter also vary from island to island and will be lower if you don't move around much, but one of the great things in the
eastern Caribbean is that the islands are so close together and there is so much variety. Things that you might want to spend
money on are: eating out, shore transportation (for us local buses but could be taxis or car rentals for some), bar bills (a lot of the social life happens in various bars), and tourist costs like tours. We tended to stock up on food and drink when we were somewhere with good (abundant choice) and relatively inexpensive
shopping. St Marten is best for this by far. The French islands are good except not so
cheap.
Grenada is pretty good for selection but
price is high. Local farmer markets can be helpful but the selection sometimes is limited. Once we were stocked up with food our weekly cost was pretty low, less than $100 a week I would think.