Very reasonable price for women's clothing, nice styles from Belorussia and other places. Also household items like towels, sheets, children's clothing, bijutery...
What to buy: Clothing, new designs for good prices
What to pay: Less than most
Its a big marketplace located in the middle of the central city. The market is known as the Apraksin dvor, and here you can find everything from cheap leatherclothing to jewelery and pirat music CDs. Watch out for pickpockets, the place can be quite crowded.
It has the traditional stands arranged in long rowes divided by long pictoresk houses keeping small specialized shops. Selling everything, ranging from washingpowder to lyxury clothing.
What to buy: Not only in this market place but in most small foodstores around the city you can find Beer and Alcoholic bewerages to a very low price. I recommend you the russian "Champagne" by "Sovietskoje". They have many different variants, so try which one´s your favourite.
What to pay: Champagne in a foodstore app. 2,5$ 750ml
In restaurant up to 10x that price depending on the class of the restaurant!
Vodka: The best is "Russkij standart" app. 15$/l. but you may find several russian brands from about 1$/0,5 l.
You see a lot of shops selling furs around St. Petersburg. Not my kind of thing, but then again, I don't have to suffer through the long, Russian winters either!