The IANA Function is NOT part to the ICANN mission assigned by the USG in the Memorandun of Understanding dated 28 November 1998.
The USG Memorandum of Understanding with ICANN and its amendments are listed at: http://www.ntia.doc.gov/ntiahome/domainname/icann.htm.
The IANA Function is subject to the Purchase Orders from the USG, which are listed at http://www.ntia.doc.gov/ntiahome/domainname/iana.htm .
Those Purchase Orders have a value estimated at $10,000 per annum par Michael Roberts in his response to Request for Quotation Number 40SBNT067020 (Proposal to the U.S. Government to Perform the IANA Function) at: http://www.icann.org/general/iana-proposal-02feb00.htm.
The initial Purchase Order is dated 9 February 2000: http://www.ntia.doc.gov/ntiahome/domainname/iana/ianacontract.pdf (photo scanned, very large 47MB file) http://www.icann.org/general/iana-contract-09feb00.htm
The current one (which expires 30 September 2002) is at: http://www.ntia.doc.gov/ntiahome/domainname/agreements/SB1335-01-W-0650-0001.htm
is an extension to http://www.ntia.doc.gov/ntiahome/domainname/agreements/SB1335-01-W-0650.pdf
This DoC document for IANA function is also listed on ICANN site at:
Its "Order for Supplies or Services" title has been renamed to "Contract Between ICANN and the United States Government for Performance of the IANA Function".