I've just come back from 3 days in St.P flying Air France from Paris (I think its a daily flight) - was €250 return.
One of the things I found expensive there was entrance fees to museums/palaces etc. Ironically, I did not have to pay at the Winter P/Hermitage as its free the first Thursday of every month. Most sights have a free entrance day but thats more pot luck than careful planning.
If Bob Aitchison was organising a summer tour I'd definately go! The advantage of going in the Winter is that there are far less tourists but, for obvious reasons, rather harder to get a feel for topography. I'd like to go next time in the summer but shudder at joining 700 other buses in the parking lot at the Catherine Palace - if you've ever been to Versailles you will know what I mean.