Edit: @potench preev.com is an inaccurate indicator of the BTC/RUB rate because it is basing its calculation "based on the USD Bitcoin price" per its footnote, so of course it will never show a big deviation from the BTC/USD rate. For the true rate you need to look at the real-time bid/ask spread on a BTC/RUB exchange such as btc-e.com which is why I linked to https://bitcoinwisdom.com/markets/btce/btcrur or you can also look at https://btc-e.com/exchange/btc_rur: bid/ask is 20855/21099 RUB as of 22:09 UTC, so the arbitrage opportunity is approximately 2000 RUB.
If people think such a scenario can't easily happen again, then they're wrong about that. A small number of major bitcoin holders mostly determine the price.
I lost a significant amount of money thinking that way during the .com bubble collapse.