Depends on the boat and the plane. Comparing a Skyhawk to a Catalina 22, the boat is way less expensive. It's less expensive to buy, less expensive to operate, and less expensive to maintain, especially if it's in fresh water. A marina slip is more expensive than a tiedown in most locales, but less expensive than a hangar. A suit of sails lasts a very long time unless you are a hardcore racer, and the rest of the rigging is durable. My father has had a couple of sailboats it the 23 foot range, and about the only maintenance that we had to pay for was to have the boat hauled out and painted every few years. In salt water that happens more frequently. Fuel burn? A Skyhawk is what, 8 gallons per hour? Our little outboard didn't consume 8 gallons per year.