The Association of Research Libraries has published a freely available PDF version of the ARL Statistics 2005-06.
The "Monograph and Serial Expenditures in ARL Libraries, 1986-2006" graph on page 13 shows that serial expenditures continue their sharp upward climb: there has been a 321% 1986-2006 increase. Serial unit cost, which peaked in 2000, has begun to climb again after a multi-year drop to a 180% increase. Monograph expenditures dropped slightly to a 82% increase, while monograph unit cost rose to a 78% increase.