Originally Posted by Avantasian

That wouldn't really make sense, cause Zurich is almost as far from Mannheim as Dusseldorf is.

Probably the cheapest way from Dusseldorf to Mannheim will be by car. There are german sites where people can offer seats in their cars for others to join them on their drive.

http://www.rideshare.co.uk/

or maybe more chances on the german page: www.mitfahrgelegenheit.de

You should be able to come from Dusseldorf to Mannheim that way for about 15 €.

I use that service on a quite regular basis from Heidelberg to Freiburg, always works well and is way cheaper than using Deutsche Bahn trains