I was an au pair in Italy for three months, and it was great (I had to come back because of trouble with the Schengen visa--being from the EU, you shouldn't have that problem)! I have an acquaintance who did it in Germany, and she loved it. Enjoy it!
My first recommendation--if you can't easily get the money from selling stuff, donating blood, etc.--would be to talk to the family about it. If the German situation is anything like the one in Italy, EVERYONE's looking for English-speaking au pairs, and there really aren't too many girls to go around. The family I stayed with was offered one application (mine) in seven months of waiting. Thus, the market is really favorable to the au pairs, and you have a fair bit of bargaining power. Obviously, you don't want to strain your relationship by asking for too much, but even if your contract doesn't state that the family pays for air fare, if you talk to them and explain that you're really excited about working with them but are having trouble coming up with the money, they'll probably be willing to either help you out a little or maybe even foot the bill entirely.