I've lived aboard full-time on a 38' sailboat for two years now. I summer in New England and winter in the FL Keys. I travel about 3500 miles per year, motoring approximately 20% of the time.
My boat-specific expenses are as follows:
General boat depreciation: $400/month. My boat is almost 20 years old. Something is always wearing out or breaking. You also have to build in depreciation on sails, motor, and all of your systems. Pay attention to basic maintenance and you'll save big $$$ by preventing problems.
Insurance: $200/month. Shop around - prices vary greatly.
Fuel: about 150 gallons - call it $500 per year at current prices
Haul-out and bottom job - $1500 every two years. I have a full keel - takes lots of paint.
Marinas - avoid them and you'll save big $$$. Buy the biggest anchor you can handle. I live on the hook most of the time. Some areas (e.g. Key West FL, Marathon FL, charge for tying up your ding ($70/month).
Pump outs - New England pump outs are free (tip the operator $5 or $10) but FL charges ($10). I get a weekly pump-out. Don't skip this - FL has stiff penalties if they catch you with an open through-hull to your holding tank.
Water - Marathon FL was the only place I ever visited in the US that charges for water .05/gallon nominal fee.
Hope this helps,