It may also be worthwhile to check out the Hertz Transfer Desk (http://www.hertz-transfer.de/index.php?lang=GB). They are specifically renting out foreign-registered cars to get them back to their home country. I don't know their rates, but would imagine they are significantly cheaper than the usual international oneway fees.

(Be advised that they've recently added a statement to their website that the service is not for private customers, but I don't know whether that's serious or just a disclaimer for legal reasons).

Maybe someone over at the Hertz forum can give you more insight if you're interested.