We cruise the PNW coast for 2-3 months every summer, the last several years primarily in Southeast Alaska. This summer we were on the water 75 days, traveled 2400 nautical miles, and while on the water spent a total of a little over $6,000, not counting food and liquor, which we would have spent anyway.
Our boat is a heavily-built 26-foot diesel powered cruiser, small enough to be barely trailerable, but big enough to sleep three adults. Well equipped for extended cruising (and fishing) on challenging waters and in sometimes difficult weather. We've had her 15 years now, and have 5,553 hours on the engine.
We generally travel at 6-6.5 knots, but can cruise at 18 when necesssary or desirable (far less than 10% of the time). We average about 4 nmpg. We don't spend a lot of money on marinas, anchoring 2/3 of the time or more. We don't spend a lot on restaurants, eating almost always on board. We don't spend a lot on boat maintenance, as I take good care of the boat and all its systems, and do almost all of the work myself.
Seems to me these are major factors in keeping ongoing costs reasonable.