For 20 years we have done one-way rentals, usually picking up in one country and dropping off in another. There were always fees, around $200 or so, which worked for us.
The last few years the fees have really gotten out of hand. 400-600 euros are typical.
We've been able to avoid some of them by taking taxis between international locations (Tivat,Montenegro to Dubrovnik, Elvas, Portugal to Badajos, Spain, etc).
In a few weeks, we'd like to pick up a car in VIE and drop it off in BER. The fees are pretty steep ($600+) and no way to avoid it with cross border taxis as we will be driving through Czech Republic.
Once or twice we've been able to avoid the fee by "repatriating" the car to the country it came from, but that was by accident and years ago.

Would like to see if there are ways to prepare for that - perhaps call the rental agency in VIE a day ahead and ask them if they have any German cars?
Has anyone done this? Any tips and tricks?