Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. It is able to keep the fizz inside, because the contents of the can are under higher pressure.
Assuming a prop, it depends on how you are propped but with a redline of 6000 rpm, most outboards perform best with a cruise in the 4000-4400 rpm range. If you want top fuel economy, get a floscan or such and see what is the most efficient for your engine and prop.
On average, the cost of a new outboard motor can be as little as $900 to as much as $25,000 or more. Take a look at the chart below to get a good idea what each type of motor is going to cost. The costs will depend on the brand, horsepower, condition and where you purchase it from.