De acordo com saint-petersburg.com :
Cash over $3000
You are entitled to import/export up to $10000 USD in cash into Russia, but sums over $3000 should be declared at customs. This applies to all foreign currencies and to rubles, with the exact quantities varying slightly from currency to currency. The most important reason for doing this (apart from your legal obligation) is to ensure that you will be able to take your money out of the country unimpeded. If you try to leave Russia with more than $3000 in cash and no stamped customs declaration, you are likely to run into difficulties unless you have official documentation proving that the money has legally been earned by you in Russia.