What amount of cash I can take with me when going from Canada to USA? I want to travel to US and want to take ~ 12000$ with me. DO I need to declare it and pay any fee?
Travelers leaving or entering the U.S. with more than $10,000 (in any form, that includes cash or checks) are required to declare it by filling out a Report of International Transportation of Currency or Monetary Instruments (FinCEN 105). This applies even if you are passing through the U.S. in transit to other countries.
In addition to Heidel's point that the US cares you are taking more than $10,000 out of the USA, Canada cares that you are bringing more than $10,000 in. The form you will fill out on arrival is at http://www.cbsa-asfc.gc.ca/publications/forms-formulaires/e311-eng.pdf and you can see they want to know if you have that amount of cash on you.
According to http://www.cbsa-asfc.gc.ca/security-securite/cbcr-dmte/menu-eng.html you don't have to pay any fee, and you are allowed to bring the money, but you MUST tell them about it. Just allow some time to have a conversation with the CBSA on arrival.