If you go through the north entrance, you can go through the first toll booth and get a ticket. I dont tell them I'm returning a rental, I just take my ticket and get on my way. I stay on the toll road until I reach the exit toll on the south end of the airport. I never get asked to pay anything. They simply take my ticket and say "thank you" and raise the gate. I then exit to the side road immediately after passing the toll to return the rental car.

It is much faster this way than taking the side roads with the lights, and it doesn't cost anything.