I feel extremely fortunate to own a boat that runs well. We've had it for eight years and do most of the work on it ourselves. We recently hauled it out and had a mechanic go over everything for a 900 hour service on the motors, bottom paint, custom dive ladder and a new hydraulic steering pump. That cost a lot, but with insurance, slip fees, fuel costs and maintenance we average about $200 per trip. We don't save much money versus diving from a commercial boat but we can dive when we want and where we want. Our gear stays on the boat so the only hauling we do is to fill tanks at the shop a block from the marina and our cameras.
Owning a dive boat is much more expensive than a car but the rewards have been worth it. I have seen animals I would have never seen on a beach dive and am able to reach wrecks and reefs that commercial boats wouldn't go to for various reasons. Some of the reefs are too small to support more than a few divers at a time.
The initial cost of a boat varies depending on how comfortable you want to be while out on the water and how long you want it to last. We didn't want a yacht with wine racks and air conditioned salons but we also wanted to be out of the elements. A pilot house is perfect for diving, as is the catamaran hull. We don't get rocked around while gearing up in rough seas. Sitting in the sun and wind takes a lot out of you, as does shivering on a windy boat on cold days. Merry insisted on a marine head rather than a port-o-potty. We also have a small refrigerator, stove/heater, queen size V-berth and hot fresh water shower. Those few luxuries make a day of diving much more enjoyable. I added tank racks, dive ladder, stainless anchor which can be seen quite a distance underwater and over 400 feet of chain so I don't have to re-splice the rode every six months. It also holds the boat closer to the dive sites so we don't have to swim as we drop. Other expenses include GPS/Sonar/RADAR and two VHS radios, one handheld. We also carry flares, life jackets, O2 and a first aid kit.
My last boat cost about 20% of the price of our current boat but repair costs were much higher. Buying as nice a boat as we could afford saved us money in the long run.
I added chain markers every twenty-five feet so I know how much I have played out.