I'm heading south from the upper reaches of Chesapeake Bay to Marathon Key, FL this coming October. My budget will likely be under $500 a month, at least until I get to the keys, where the cost of everything is high. Hopefully, I'll be able to find sufficient work ,and do some horsetrading, to support my mooring and dockage fees while there. I think that without the mooring and dockage fees $500 a month would be a piece of cake, even for a couple.
A big chunk of the budget would be for food. Well, at least for most folks. Hell, you're on the water and if the water's fairly clean, you can catch dinner most nights. Breakfast is cheap--some Egg Beaters, a slice of American cheese, a couple strips of turkey bacon (or the real stuff), an English muffin and a cup of coffee makes for a huge breakfast.
Lunch involves a couple slices of bread, a couple slabs of bologna (or any other lunch meat), a slice of your favorite cheese, some mustard or mayo and a cold drink.
Dinner can be pretty much anything that suits your fancy. If you're down in the sunny south, beer battered grouper fillets would be tough to beat, especially when served up with some fresh, steamed broccoli, some fresh-baked garlic bread and an ice-cold Margaretta. OH YEAH! Of course you could substitute the grouper fillets with fresh-caught lobster (when in season) broiled on your gas grill and served with drawn butter.
Grouper have a relatively short season these days, but there is no season on French and flannel-mouth grunts, which is a smaller member of the snapper family. I've caught them on a small strip of red cloth attached to a wide-gap hook and jigged close to the bottom along channel edges of the keys. The meat is snow-white, sweet and delicate.
If you get tired of seafood, which most of rarely do, you can always make a homemade pizza. All you need is some basic ingredients--flour for the crust, some spaghetti sauce, toppings, cheese and wallah, you're living high.
If you have a tight schedule, then fuel will be a significant expense--and it's gonna' get a lot more expensive in the very near future if the present administration has its way. However, if you don't have a tight schedule, and don't have anyplace in particular where you must be, then fuel doesn't play much of a role in your overall expense.
Repairs and maintenance will vary, depending upon your ability to fix things without the assistance of a repair facility. When I was young I was able to rebuild engines, leap tall buildings with a single bound, lots of neat stuff. Now that I've grown old I no longer feel comfortable going up the mast or squeezing my bulging frame into the engine compartment. Consequently, I leave those jobs to younger folks that hopefully will not overcharge me and know what they are doing.
There's lots of options for you to consider when setting up that budget and the best advice I have is to look at all of them carefully, then determine which items are necessities and which ones are just things you would like to have, but really don't need in order to be cruising.