Since most invoices from law firms are based on hourly billing rates, corporate law departments should consider studying the rates they are paying.
Take the five law firms you paid the most in the past year. From each of them record the hourly billing rate of the five lawyers who billed the most hours, keeping in mind that you want data from several different matters. Compare those billing rates to average billing rates as provided by third-party surveys. Although not precise, this analysis can at least show you where your preferred firms stand in terms of their hourly billing rates.