We are on Crown Princess and will be in St Petersburg 11/12 Sept. We have booked a two day tour with Alla Tour (www.alla-tour.com) and have been quoted $300 pp. Hope this helps.
And we paid $325pp for a small Comfort tour with Alla-tour in July and would recommend her highly.
We have just been on this cruise to the Baltics on the Celebration. Half day trips were approx £30 per person. You do of course need your own visa if you want to organise your own, as I'm sure you know. Enjoy your holiday, we certainly did.