The following items are taken from an article in Baseline Magazine:
70% TO 85% OF ALL PROJECT REWORK COSTS are due to errors in requirements, says a 1997 article in American Programmer.
30% TO 50% OF THE TOTAL EFFORT EXPENDED on a software project comes from rework, according to an estimate by Borland Software.
ALMOST 50% OF DEFECTS IN SOFTWARE PROJECTS can be traced to errors in requirements, Borland says.
71% OF FAILED SOFTWARE PROJECTS suffer from poor requirements management, according to a paper presented at the 1999 Applications of Software Measurement Conference.
These numbers sure sound right to me.