here you can check the prices and offers if 1 GB is enough:
Ergebnis Mobilfunk Datentarife - teltarif.de
and here if you need more (i assumed 5GB):
Ergebnis Mobilfunk Datentarife - teltarif.de
(make sure that you look at the "Prepaid" offers only, not "Vertrag" = contract)
you can see that the offers are mostly the cheapest on Eplus network, but they have the smallest 3G coverage. Best coverage is by far on T-Mobile, so the prices are also much more expensive.
Chris