Rent/buy a bicycle. Take the train to Passau with Bayernticket and ride along the Danube bike trail. Cost: depending solely on the distance you manage per day and how much you spent on accomodation.

Another option could be to cross the border into the Czech Republic and take a Czech train to Bratislava.

http://www.jizdenka.cz - for price
http://www.bahn.de - for schedule and route

http://www.toytowngermany.com/forum/index.php?showtopic=67188

If this is too complicated, check the bus:
http://www.touring.de