Hm, I just rented a car from Dubrovnik to Zagreb (Croatia) and didn't pay a drop-off fee. I rented from National, via the internet. The rate was actually fairly good, about USD 280 for five days. Considering the sorry state of the USD right now, I thought this was reasonable. Try National's website ("worldwide locations") and see what you get:

http://www.nationalcar.com/servlet/D...4bd49b233b7bb7