If airline behavior is consistent with the models I teach students (in intermediate microeconomics), then airlines should equalize their rate of return on invested capital across all routes they serve. If one route is consistently more profitable than another, then airlines should invest more in the more profitable route (bigger planes, more frequent service) at the expense of the less profitable route.

There are exceptions to this rule. If an airline has monopoly power in a certain market (and no other airline can credibly threaten to enter the market), then rates of return will be permanently higher in that market. Also, rates of return should be higher in slot-controlled airports (LGA, JFK, DCA, ORD in the US) since there are barriers to entry. Similarly, when governments restrict the number of carriers or flights serving a particular market, we expect profits to be higher.