Any travel agencies do just Moscow/St Petersburg, with options to extend stay?
how much would it cost?
Both cities can get around with their metro. Most sights are within walking distance of the metro as well. Just need more time (looking at the metro stop name) with the Russian alphabets (Cyrillic script).
Between St Petersburg to Moscow, you can take the express train or the overnight train.
The value of Rubles has dropped a lot over the years. It's not as expensive as before. A MacDonald's meal cost RUB221 / SGD9.20 in 2012. Today, RUB221 is S$5.30.