I flew to Moscow in 2007 with my C182. The route was UK, Paris, Prague, Vilnius, Moscow, St Petersburg, Tallin, Malmo, Groningen, UK.
I went VFR as far as Vilnius and then IFR until Groningen. There's no need for a Russian nav when IFR, not sure about VFR. The terminology is a little different, and obviously they use metres instead of feet for altitude.
I landed at Zhukovsky during the MAKS show. Avgas was available at €3 or €4/litre, not sure about the other Moscow airports. St Petersburg didn't have Avgas, and weren't hugely interested in GA traffic, they would only accept an arrival after midnight, which made for an interesting flight.
The fees, landing, handling, lights, and all sorts of other bits and pieces came to over $1,000 just for the two Russian airports. They also love their paperwork, and I spent at least seven or eight hours at each getting all of the paperwork sorted. (I also got a bonus day in Vilnius waiting to get in, thanks to more Russian paperwork).
If you want more specifics, drop me a PM or email (or pop over to Bath for a pint)
Ian
PS There was no way that we could get permission to fly over Belarus