[Verger99]

Not sure if this helps, but here's what I found:

The earnings from issuing both coins and currency reduce government borrowing costs; however, how these earnings are budgeted and accounted for differs. Production costs of coins and currency are generally treated the same in the budget and accounting statements. The difference between the face value of coins and the costs of minting them results in earnings, called seigniorage, which is shown in the budget as a reduction in needed borrowing for the government, after the deficit or surplus for the year is calculated. The budgetary impact of seigniorage is interest avoided from the borrowing it displaces and is not visible because it is neither quantified nor shown in the budget. The government also generates earnings by issuing currency, but it is handled differently. The difference between the face value of currency issued and its production cost goes to the Fed. The Fed buys collateral, usually Treasury securities, to back up the currency issued. The interest collected on those Treasury securities is used to pay for Fed costs, and the remainder is returned to Treasury. The budgetary impact of issuing currency comes from the interest returned by the Fed, which is shown as a budgetary receipt and counted in the calculation of the deficit or surplus. Production costs of both coins and currency are shown as costs of operations in Treasury’s financial statements. According to the Federal Accounting Standards Advisory Board, seigniorage should be shown as a source of financing in Treasury’s statement of changes in net position, whereas interest returned by the Fed for currency is shown as revenue in Treasury’s statement of custodial activity. Treasury has not been reporting seigniorage this way but made the correction beginning with its fiscal year 2003 financial statements.

http://www.gao.gov/assets/250/242107.html