one alternative to Lappeenranta: Tallinn (capital of EU-member Estonia) is a 3.5 h drive away from St. Petersburg. Tallinn is served by two low cost carriers: EZY from Stansted and Berlin and Flynordic from Copenhagen and Stockholm.
There are some low cost operations from Germany to Russia:
DBA (formerly Germania Express) flies from Berlin, Düsseldorf and Munich to Moscow (DME)
German Wings recently announced that they will launch Berlin-Moscow in summer, after granting of traffic rights. May be it isn't that difficult getting traffic rights to Russia?