Tampere is located in Finnland and can be reached quite cheaply by Ryanair  from Frankfurt Hahn. Then continue to St. Petersburg by train or bus.
If you take Ryanair, you initially depend on the shuttle bus to transfer you from the rather distant-from-town airport Tampere, taking you to Helsinki directly after arrival for about 25 Euro. You might as well spend the night in Tampere and then take off from there by train, heading to St. Petersburg in the morning. In this case take the bus to town.
In case you arrive in Tampere with Ryanair at 21:55 in the evening, you won’t have a chance to get to St. Petersburg at night without sleeping over. One exception: you speak (or someone you know speaks) a little Russian, in this case you can buy a ticket for a Minibus for circa 50 €, taking you directly from Tampere to St. Petersburg. This Minibus is used by Russian travellers and the very few foreigners who know about this service from Tampere airport directly to St. Petersburg. The operating company’s telephone number is +7-812-3102837 or +7-812-3194011. but as stated before: They speak only Russian.