Are you talking about the big consulting firms, i.e. Accenture/IBM? The don't pay by percentage of billing rate, they have a pretty complicated billing structure with all sorts of internal rates, blended rates, rack rates, etc. It's pretty confusing, and their margins seem huge at first glance but they have so much overhead it's crazy.
If you are considering going into a big firm (which can be very good idea depending on your career strategy) you should really try to assess the opportunities for how much benefit they will bring to your career rather than just pay. At a place like IBM you won't earn much as a junior, but you might get exposed to lots of things that you wouldn't be otherwise, and get tons of training, etc.
If you are more senior, you can look at the firms as a fit for your skillset. The more work they have in your area, the more you'll ultimately earn.