Living in Germany is not expensive compared to other European places. The costs of living, however, differ greatly from region to region. The decisive factor is the cost of accommodation and transportation, while food prices differ at most slightly. While it is possible to rent a good accommodation in one of the smaller towns for 100 EUR / month, a simple room in the cities of Munich or Hamburg can cost you up to EUR 500 / month (if you find one at all). The cost of public transport, that often overcompensates the advantage of cheaper rents in the suburbs, must be added. It is highly recommended to carefully check and resolve the issue of accommodation and transportation prior closing the internship contract.
In general, a good orientation for interns are the figures given by the German Academic Exchange Service (DAAD) for students (ignore the chapter on education expenses, but add some 25% to the budget lines for culture and transport since interns usually do not benefit from the student discounts):