The primary method of valuation is the Transaction Value Method, and there are six additional methods to consider if transaction value cannot be applied. Allowable deductions from the selling price include freight, insurance, and customs duties, if included. Required additions to the selling price include assists, subsequent proceeds to the vendor, and selling commissions. To avoid penalties that would result from a U.S. Compliance Review, importers should ensure that their valuation methods are correctly declared.

http://www.cbp.gov/xp/cgov/import/informed_compliance/