I'm sorry that I write in English, but my German is too poor to write posts.
That's strange that you can't find anything for less than 1400 Euro. I've just checked one of Polish websites dealing with airline tickets (http://rezerwacje.bluesky.pl
) and found lots of offers for approx. 850 Euro. For example, there's a flight from FRA to MRU with Condor, on the 5th of December, coming back on the 27th of December and it costs 3.385 PLN (which is approx. 845 Euro) incl. taxes and service charge. Well, since I haven't tried to book this flight, there's a risk that this offer may be found not valid any more once you go to booking site, but it's worth of trying.
In Germany I'd recommend expedia.de
Best regards and good luck with searching!