The Association of Research Libraries has released ARL Academic Law Library Statistics 2007-2008 (print version also available).
Here's an excerpt from the press release:
The Association of Research Libraries (ARL) has published the ARL Academic Law Library Statistics 2007-2008, which presents data that describe collections, expenditures, personnel, and services in 75 law libraries at ARL member institutions throughout North America.
In 2007-2008, the reporting law libraries held a median of 345,935 volumes, spent a total of $215,630,657 and employed 2,129 FTE staff. Expenditures for materials and staff accounted for the bulk of total expenditures, at 47% and 45% respectively. Respondents reported spending a total of $20,345,053 for electronic materials (this was 22% of their total materials budgets); this includes a total of $17,200,532 for electronic serials.
See also ARL Law Statistics Tables 2007-08 (XLS file).