According to the current State-of-Hesse legislation tuition is basically not charged to students regardless their nationality. Only some Master programs charge tuition. Please turn to the single program description for the amount you will have to pay.

The fees relate to administration costs, welfare contributions (subsidized meals, liablity insurance on campus, student union and the like) and to the socalled semester ticket. The price of the semester ticket is the biggest portion on the bill. The ticket (=student ID plus passport) allows for free rides in the public urban and regional transportion system for an entire semester.
The amount of fees varies over the time. Currently students have to pay approx. € 250 per semester.