One Web site that may be helpful is www.RealRates.com. It gathers data on what contractors are getting paid per hour, cross-referenced by various criteria including whether the job is brokered or non-brokered. It's based on data that site visitors volunteer, so it's more anecdotal than scientific, but it can still be informative.
ComputerJobs.com also has hourly rate survey information, though I don't think they specifically analyze broker mark-up as a factor. See http://www.ticker.computerjobs.com/content/ticker.aspx?jcid=107&type=2 Mike Smith
