Assuming your boat is paid for and fitted out, how much does the cruising lifestyle cost? Well, that depends on a number of things, but I can tell you what it cost us. Our budget for a year's sailing was 5000 pounds each, which was to include EVERYTHING, and in the event, it turned out to be exactly right. In fact, it covered a number of things that we had never budgeted for, including 1000 pounds worth of engine repairs in Barbados, three diving courses each (plus extra dives), purchase of two sets of diving equipment, and car or bike hire in various places.
Naturally it also included food, including a number of meals out, plus copious quantities of alcohol, cigarettes etc. While in Europe we had to pay mooring and Marina fees in some places, which are also included.
A great deal depends on where you are. We didn't visit that many islands, but the contrasts in prices were amazing. The St. Vincent Grenadines seemed horribly expensive - I think the prices must have been geared to the charter boat market. Trinidad and Tobago are extremely reasonable. Grenada and Barbados are somewhere in the middle. Venezuela has the reputation for being very reasonable, especially for beer and diesel.
In general, if you steer clear of expensive imported goods and eat, drink and smoke what the locals do, your money will go a long way. In the Caribbean this means you drink a lot of rum and beer, and eat a lot of fish and chicken.
We took the decision to go half-and-half on virtually everything. This makes accounting a lot easier, and the differences would have been fairly trivial anyway. The only exceptions were items bought for strictly personal use, which were paid for separately.