A report from BNA, HR Department Benchmarks and Analysis 2008, found the median per capita budgeted expenditure for HR departments was $1,082 per worker in 2006. The counterpart for legal would be total legal spending per corporate worker. I have never seen that figure calculated and analyzed, probably because it neither gives guidance nor allows equivalent comparisons (See my post of April 18, 2009: lawyers per 1,000 employees with 6 references.)
The study, conducted from in mid-2007, gathered usable data from 607 respondents, some of which were in companies with multiple HR departments. The full report is available for purchase from BNA PLUS at (800) 372-1033 and it may suggest other improvements in the arena of legal department benchmarking.