There's not really any good reason to expect arrival costs to be the same as departure costs for a given location. For instance, it takes much longer to board a

There's not really any good reason to expect arrival costs to be the same as departure costs for a given location. For instance, it takes much longer to board a plane than disembark, which means there's a comparative premium on leaving from less efficient airports. So even if it weren't about maximizing price, you'd expect different costs for different directions.