There is a nice big room available in a 2-room apartment for long-term rent in the historical center. For the price of hostel you receive a separate room, shared kitchen and bathroom. There is another girl living in the apartment.
The room and the apartment all together are Soviet style renovated, but it's equipped with everything necessary - washing mashin, wi-fi ect.
The house is located 5 minutes walking from Chernyshevskaya metro. It's about 30 minutes walking to Hermitage and Petro-Pavlovskaya fortress. There is a nice Tavricheskij garden nearby, fitness center and a lot of restaurants for all tastes and wallets.
Preferably for a 2 or more weeks rent. Only for one person.