Dining out in St. Petersburg is not hugely expensive by European standards, but nor is it particularly cheap. Filling, good-value meals are readily available in any number of Caucasian and Central Asian restaurants, but finding real budget options takes quite a bit of effort and imagination.
What St. Petersburg really lacks in this regard is much in the way of reasonable-quality street food. Pancakes and pirozhki (buns stuffed with sweet or savoury fillings) can still be found, but either in a few surviving Soviet-era venues that have gained a cult popularity, or in local fast-food chains. Otherwise, budget restaurants in St. Petersburg are exceptions to the rule.