Should be a flat 900 or 1000 Rubles. After exiting immigration and customs, stop by the official Taxi Pulkovo booth, prepay your flat fare, you'll get a receipt with your taxi's make, model, and license plate number, exit, turn right, find your assigned taxi, and off you go. Piece of cake and hassle free. Just did it myself last month.
http://www.pulkovoairport.ru/en/transport/taxi/
On the way back to the airport, booking with Taxi 777 the day before is a good option. They respond to booking requests via email promptly in English, will also quote you a flat fee, and can arrange a specific pick-up time at your hotel. I paid 700 Rubles to get back to the airport from the St. Isaac's area. Again, no rip offs and very easy.
http://www.777taxi.ru/english.html#cont