Money and Other Monetary Instruments
You may bring into or take out of the country, including by mail, as much money as you wish. However, if it is more than $10,000, you will need to report it to CBP. Use the online Fincen 105 currency reporting site or ask a CBP officer for the paper copy of the Currency Reporting Form (FinCen 105). The penalties for non-compliance can be severe.
"Money" means monetary instruments and includes U.S. or foreign coins currently in circulation, currency, travelers' checks in any form, money orders, and negotiable instruments or investment securities in bearer form.