Go for Moscow or St.Pete's. In Moscow it's possible to find relatively cheap places to live if you collaborate with smb and share a flat. St.Pete's is much cheaper but not less amazing. You could travel and see other cities, but I recommend sticking to those two. Vibrant, colorful and full of life. From your list I'd pick Kazan. Maybe Sochi? (too expensive though) or Krasnodar?