RUSLAN 2 - THE CUSTOMS DECLARATION
For up to date information, try the links below.

Standard advice is that we do not need to fill in a declaration unless we are carrying $10,000 or more in cash. However, you may not be allowed to take more than $3,000 out again if you have not declared it on entry! So if you have a large amount of cash and / or valuable personal jewellery etc., fill in a declaration when you arrive, have it certified in the red channel and keep it until you leave (advice from 2011).

https://www.iatatravelcentre.com/RU-Russian-Fed.-customs-currency-airport-tax-regulations-details.htm
http://www.saint-petersburg.com/russian-visa/migration-cards/customs-declaration.asp