what EXACTLY are the fees for taking a rental car out of the country

Did you try to book via Kayak.com? Checking just now, for example pick up the car in Dubrovnik and drop it in Ljubljana, I'm referred to Sixt.com (big rental car company, used them twice over there) and get a quote for Oct 7 to Oct 14 2014 of $11.09 per day + $14.72 per day "One-way fee" and other fees...total of $209.00 plus $52.24 US tax = $261.25 for 7 days or about $38/day.

You could also try gemut.com They book with various companies and will email you a quote. I just booked a car with them for Spain in February.

It is possible that the Dubrovnik - Ljubljana route is popular in both directions and so there are no big drop-off fees.

The no-drop-off fee is valid across all of Croatia. We picked up our rental car in Split, drove all around Croatia and Slovenia, returned the car in Opatija and took an easy train back to Ljubljana (then onwards to Vienna and Budapest). Zagreb also makes a good drop-off point, with many train options. We also used Sixt and were very satisfied.

It is usually no problem to take a rental car "out of the country". It is common practice in the EU. Dropping it off at a different location in the same country is also usually not a problem.
What costs is dropping it off in another country. Most countries only allow allow cars to be rented out which are registered, and have paid tax, in that country. So if you rent in country A, and leave the car in country B. It cannot be rented out in country B, but has to be driven (by a paid employee) back to country A.

The easier way to figure it is a variation of Andrew's method.

Use Kayak and make a surface run (without switching to an individual agency). First go-round is picking up and dropping off in the same city. Second is using two cities in different countries. Keep everything else constant. The difference is the one-way cost.

Just rented at Zagreb, drove through Croatia and Slovenia. Dropped off at Dubrovnik. 8 days no drop off charge. Fleet charged about 50 Euros for crossing the border into Slovenia. Don't forget Vinjetta in Slovenia, 15 Euros. If I picked up vehicle in Budapest and dropped off in Dubrovnik the drop off fee would have been $600.00. Took the train from Budapest to Zagreb. Rental was with Fleet through Autoeuorpe. Autoeuorpe and Fleet customer service was great. Toll roads in Croatia and Slovenia are great but the tolls were expensive.

Andy

Thanks Andy! That helps a lot and is consistent with what I had heard (that renting in Croatia and dropping off in another country could add upwards of $500 to overall cost). I guess I will try to drop off in Istria somewhere and then take public transit across the border and rent another car in Slovenia. What a pain...oh well, that's how it goes and I'm sure it will all be well worth it in the long run. BTW, what did you think of Slovenia and Croatia...any MUST SEES in your experience?

Thanks,
Kaye