My wife and I moved aboard our 41 ft. sailboat last fall. In addition to articles in various mags that help with cost models, you can make one of your own. I remember a very good article by Leonard (I think) which profiled costs in three categories which you can use to calibrate against your own lifestyle. Use their categories as a starting point and add important items of your own. Then, get specifics for yourself. Copy your food budget into the spreadsheet, same for entertainment, etc., unless you plan to make a lifestyle change. Consider your onboard equipment carefully, you will be anchoring out a lot and dinghying around for provisions and to sight-see.
If you start your own spreadsheet, you can modify your items and costs as your research progresses. However, the cost of unforseen events, like the nearly 100% cost increase in diesel fuel, is difficult to predict. So we have a little 'nest egg' for contingencies that is not used for operating expenses, it's for repairs, equipment replacement, etc. For us, food costs a little more than we planned, and we motored a little more than we planned. I think if you develop a basic budget and tweak it as you read these articles, you'll get pretty close, and when you finally start living aboard you'll do what everyone else does -- adapt. Happy sailing!