For a lot of international students this is the most important question. As you’ll understand this question is not so easy to answer, because prices differ from city to city.
As in every country of the world, the bigger cities have a higher price level than the smaller ones. And there are cities and regions that are most popular, while others – let me put it this way – are still insiders’ tips.
Also there’s always someone who will do it for less, but you should look at what you really get for the money. If you have to pass a language exam for university admission it is most important, that you find a language school that has bullet proof expertise ;-) in preparing students for the German language exams TestDaF or DSH.
Where you can find such a German language school I described in my post “Where can you learn German in Germany?”
With this in mind you should calculate between 3,50 Euro (reasonable price) and 5 Euro (high price) a lesson for an intensive German language course without any additional services like airport transfer, accommodation, meals, activity program and so on.
As I quoted in my post “How long does it take you to learn (enough) German?” it will take you between 150 (fast learner) and 200 lessons (average learner) per level of the CEFR. That means between 525 (reasonable price, fast learner) and 1000 Euro (high price, average learner) per level.
So if you start from zero, between 1575 Euro (reasonable price, fast learner) and 3000 (high price, average learner) to reach B1 and between 2625 Euro (reasonable price, fast learner) and 5000 (average) for C1 of the CEFR.
That is certainly an investment, but you should keep in mind, that after passing the German language exam you can study for free (!) in every public German university.