thanks josebrown. well, i don't pay anything for the flight anyway (my mum works at lufthansa and gives me a free ticket) and i found a place to stay (a nice person who'll lend us his couch) so this will cost nothig, too. so just for food, metro, enty prices etc do u think 400€ (that's abot 550$) will be enough for two weeks.
by the way I had a very good time travelling europe last summer, spending very less money. but well, i spoke the languages, i knew the culture and people under 25 get free in every museum..