There's no limit. If someone asks for ridiculous amounts of cash, chances are they are terrorist-wannabes who just want to kill you.
In the Benitez, I have tried to encourage ethical piracy. That is, taxation that will not cripple traders. ie. we ask for 2 million per pass from a trader with 3000 cargo, and tax players with less than 3000 cargo hold only 1 million credits. There are exceptions (some commodities are encouraged, not taxed, and some commodities are confiscated).
The system described above allows me to tax and maintain a clear conscience.
Speaking of which, if a Benitez taxes a trader and he thinks it's an unfair ammount (non-rp), please contact me or post on our forums.