Steve was right the design and the complexity does play a large part of the picture but so does the CM's own capabilities, equipment used, etc.  will make the variations between house  to house enormous.  Not to mention the management philosophy, labor rates per hour, material purchasing power etc.  

For the CM's you are considering ask what equipment is used, look into the per hour placements and the general machine bells and whistles.  If you go on a tour look at the amount of product in wip (the more there is the more overhead you will generally pay for), look for a CM that isn't afraid to let you look at the run time and data.  If the CM you choose does a good DFM process you will know what the standard run time is and then also the added charges due to layout/design issues.  Don't get in to any CM who won't be open about their process and run time with you. Another technique I used a lot when I was in the OEM business was to go to the CM on-site for a production approval lot.  I watched each process, timed it, and also recorded all defects that were found.  It can be a small lot but it does make a questionable CM very honest.  

Kathy