Mark
I checked the rules on registration of visa and it seems they changed about a year ago. Now, the responsibility for registering a visa falls upon the receiving/inviting party - ie the people or company which issued your invitation, this applies only if you're staying in private accommodation, if you're staying in a hotel the responsibility falls upon the hotel. However, if you then visit another part of Russia for more than three working days, the visa needs to be re-registered. If you then go back to your point of origin it needs to be re-registered yet again. You can find the details
here
The only hotel I've stayed at in SPB is the Nevsky Palace, I don't remember exactly how much it cost but it was an astronomical amount. If you can find somewhere for $100 per night then you've done well, 10kms is a long way from the city but if it's close to a metro station then maybe that's ok.
Next task is to confront the massive Russian bureaucracy to get our visas. I have to say, with all these payments for the "invitation," the visas and to register once there, I can't help having the word "scam" come into my mind. Seems so many have their hands in my pockets already, and I haven't even gotten there yet.
Welcome to Russia ! Trying to separate you from your money is an art form in Russia