SDR's/XDR's or Special Drawing Rights are virtually a currecy of their own and do fluctuate.
http://www.xe.com/ucc/convert.cgi
Live rates at 2008.07.15 04:35:13 UTC
1,000.00 XDR = 1,639.36 USD
International Monetary Fund Special Drawing Rights ---- United States Dollars
1 XDR = 1.63936 USD 1 USD = 0.609994 XDR