According to a piece by Julius Melnitzer in the April 2005 issue of Corporate Legal Times, “the United States exported $3.38 billion in legal advice in 2003, almost four times the $879 million it imported.” The statistics come from the Department of Commerce.
If that means non-US clients paid US law firms the $3.38 billion, and U.S. clients paid foreign law firms the $879 million, it would seem probable that law departments intermediate a large chunk of those purchases.
I marvel at the methodological complexity of arriving at the figures. Truth be told, my metrics meter has gone deep red.