While the number is certainly debatable and it will vary by boat type, condition, age, etc., a
VERY rough rule of thumb that's useful for getting an idea of what a particular boat will cost you to own is to figure the annual cost of ownership--- moorage, insurance, electricity, water (maybe), fuel, maintenance, repairs, upgrades, new equipment, labor (if you hire work out)---- basically everything except the purchase payments if you finance the boat--- is ten percent of the purchase price of the boat for every year you own the boat.
Some years will be more--- like when we had to have our worn out engine mounts replaced or later when we had one shaft straightened and bought a new one for the other side, or when we had the old failing exhaust systems on our engines replaced with new custom-built fiberglass systems--- and some years will be less. But while we have not actually sat down and run all the numbers, a rough estimate shows that over the last 14 years we've probably averaged pretty close to that ten percent figure.
Actually, if we did run the numbers I suspect we'd find it's probably more like fifteen percent these days as moorage prices continue to creep up along with labor (diesel mechanic, prop shop, etc.) and of course, fuel although compared to everything else fuel is a very small percentage of the annual ownership cost.
So as Al rightly says, don't get caught up in the euphoria of "I bought a boat, now I can go boating" sort of thing. That statement is true, of course, but make sure you realize if you don't know it already that what the purchase of a boat has really done is admit you to the world of supporting your local economy by paying and paying and paying and paying for the privilege of continuing to own and use that boat.