Probably depends on the hotel and their markup. I buy my tickets at the station or at any high street TA. Fares for tommorrow, for instance, start at R3081 one way - that's for the fast train.

Note that the Russian Railways have a system of seasonal markups. If the markup for the dates you intend to travel is positive - like in the summer - you can avoid it by gettting your tickets from Deutsche Bahn. But then you'll have to be at the station some 30 minutes before departure as the train attendants have a hard time trying to figure out if those are valid on their trains (they are).