There are a couple old cruising reference books that are old but still salient to the planning process today. I've recommended both several times. The first is Annie Hill's Voyaging on a Small Income
and the second is the Pardey's Cost Conscious Cruiser
. Both books encourage cruisers to think about long term sustainable living on a boat. I found Hill's book to be very useful in terms of weighing options for type of boat, capacity for stowing gear, etc. The Pardey's tend towards the "you can do much with very little" mentality which has worked for them but may make many people very unhappy. Other sources of more current cost information include the Bumfuzzles sailing blog (look at their 4 year circumnavigation, though they had a big budget each month, they were very honest about sharing their cost information. They were also exceedingly inexperienced and open about the matter and shared that information, too. Throughout, they often took honest assessment of less than stellar sailing skills and let the world know all about it. Very gutsy.)
If you don't use Quicken or a similar program to keep track of your costs, start to do so now. Understand your own spending habits, wants and needs before you take off.
Some generalities written by experienced cruisers:
If you spend a lot on something now (say booze or entertainment) you're going to spend a lot on the same thing while cruising. We don't change overnight into thrifty monks.
Take what you spend on shore for food and increase it. Don't expect it to go down. On average it will not.
Take what you expect to spend on boat repairs and double or triple that number. (Here, I suggest you talk to a knowledgeable marine surveyor about the expected cost of maintaining your vessel for many years in the same good shape you bought it in.)