Well picture how much money Microsoft would make if Windows cost $1000000000000000000000000000000000. Nobody would buy it. Essentially, when you have a monopoly on the route, you want to charge the price where margin demand = marginal cost. Your ideal level of profit comes when you can raise the price to the highest level possible while still keeping your plane full.
Take a course on it if you're in high school or do some basic economics courses if you're done school. Learning about pricing structures is interesting.