The easiest way to get to St Petersburg is by plane directly to airport Pulcovo 2 which operates beside all major companies also budget Skyexpress, Germanwings,Windjet, Airberlin.
You can also fly to Helsinki or Lapeenranta in Finland and take a bus or train from there. Train costs around 50-75 Euro, bus 35-50 Euro, small private charter buses around 10-15 Euro. Charter buses are privatly owned minivans which leave from Finnkino in Helsinki and arrive in 7-9 hours at Oktyabrskaya Hotel on Ligovsky Prospect in St Petersburg. Be prepared for shopping stops and long customs if you go for this.
Another way to come relatively less expencive to St.Petersburg, were to fly with a discounter to Riga or Tallinn and take a couch to St.Petersburg from there.This route is operated with Luxexpress and Eurolines, price is around 35-50 Euro. Delays at customs are possible during high season.
Helsinki and St.Petersburg are also connected with a ferry line.
If one can get a good price for flight to Moskow, there is a possibility to come to St.Petersburg with an overnight or express train. Price may vary from 30 to 150 Euro.
Check current arrival-departure situation in Pulcovo2 Airport.
List of all air-companies flying to St.Petersburg