*Current* cheap non-terrible-schedule roundtrip airfares from various European cities for the @party weekend based on a quick search on ITA Matrix, in euro (converted differing currencies with oanda.com):

Hamburg – 468
Madrid – 469
Brussels – 483
Amsterdam – 488
Frankfurt – 489
Budapest – 511
Warsaw -522
Helsinki – 591
London – 643
Stockholm – 653
Oslo – 660
St. Petersburg – 673
Paris – 682
Copenhagen – 700