Avis often quotes without one-way fees (except on a lot of corporate contracts). They just jack up the daily rate. So you really need to look at the full cost, not just one-way fee.

And the fee/costs vary widely. There's not one set rule for fees that I'm aware of. Depends on the supply/demand and many other factors that change often. For example, driving I to FL in the fall from the northeast is often cheap. Driving out in the spring in the opposite direction is the same (rates often <$20/day. But going the opposite direction as they want can yield costs that are 5x higher, so there's not a blanket rate/fee.