I have found that if I walk up to the counter and try to book a car one way it doesn't go well. Similarly if I call a particular location it is generally a problem. They seem to always either have no cars that they can let go one way or have a big surcharge.
Funny story about rental cars: A friend was flying back home, and his flight to Atlanta was delayed so much he missed the last connector. He went out to the rental car counter about 11:00 pm and tried to rent a car one-way, about 200 miles; nothing was available. So he went back into the terminal, called the national reservation office, and there was a $250 surcharge for one way rental that day. He asked if there was a car available for tomorrow (starting about 40 minutes from then), and wouldn't you know, it was a lot cheaper. Finally he went back out to the rental counter and asked if he could pick up his reservation early; no problem. He made it home for $80.