Germany is the place to rent the car if you plan to drive from a western European country into former Eastern Bloc countries. For example, this December you can rent a compact car in Germany for a week for $152 and drive it in Zone 1 eastern countries (Czech Republic, Poland, Croatia, Slovenia, Slovakia, Hungary). Travel east from other countries is either prohibited or more expensive. The east travel rate for the same car from Vienna is about $275, from Zürich $273, and from Venice $353. With a few exceptions east travel is typically limited to Fords, Skodas and Opels, so you can forget driving a BMW, Audi or Mercedes into Zone 1 countries.
Higher prices and more restrictions apply to Zone 2 countries such as Romania, Serbia, and Bulgaria. Get a written quote.