What is better? An I/O, an inboard, or an outboard? What’s cheaper?
The best answer is… It depends. I/Os are the often the least expensive, and you have the advantage of having a realtively car like (simple) installation. You can get to the engine pretty easily in most boats. The disadvantages are that the outdrive can’t tilt clear of the water, so if you boat in saltwater the aluminum outdrive is always submerged and subject to corrosion. Also, the rubber bellows that encloses the drive shaft, as it comes out of the boat and into the outdrive is usually at least partly submerged. A hole, or tear, in it can sink the boat.
An outboard is probably the most expensive, can be tilted clear of the water, and has the best horsepower to weight ratio. The disadvantages are that outboards have become as complicated as spacecraft and unless you are a good mechanic, you can’t do much if any, of the maintenance yourself. 2 cycle outboards, where the gas and the outboard motor oil are mixed together almost all have automatic mixing pumps (above 40 HP or so) that take care of this chore, but have been known to fail, with catastrophic results. Some folks disconnect the pumps and mix the oil and gas manually. The new 4 cycle outboards may have it all in one package. They are powerful, quiet, have outboard convenience and are now available in high power versions (200+ HP). However they are very expensive and heavier than 2 cycle engines.
Inboards probably have the best combination of engine placement, inside where it’s accessable, farther forward for good weight placement, and weight (they can be lighter than I/Os). The disadvantages are that the prop is fixed, is deeper, and can’t be tilted up like the outboard or I/O. Also it is often hard to trailer an inboard since the trailer has to be higher to accomodate the prop/rudder. Also handling an inboard in reverse takes more practice since the prop is fixed and the rudder is not effective until water flows over it, either from prop blast or motion.
So you have to choose based on the type of boating that you do, your budget, and also what is popular in your area (this is important when looking for a mechanic). All 3 are fine and service millions of boats each, but one is probably better for the type of boating that youdo.