Don't get confused about what a glass master is. It's not made at mastering (technically "pre-mastering") houses. It takes expensive and not commonly available equipment and is the first step before making stampers at the manufacturing plant. Though people frequently misues the term, you do not get true glass masters from mastering studios. Take a look at the following links (or search for LBR & CD manufacturing) and reconsider whether anybody is actually supplying you with a glass master, and whether it would be a good idea even if they could:
http://www.mfdigital.com/2004/06/cd-...aser-beam.html
http://www.roxio.com/en/support/discs/cdrmfg.html
As for why the master CD costs so much at these top level houses, it's because of the time it takes to make it with all the QC (Quality Control) steps incuded. It's typically burned fairly slowly (no 52X mastering!), then put through a machine for error testing, sometimes loaded back, verified, and perhaps occasionally null tested, Start IDs checked, and finally listened to all the way through. It's usually a several hour process, and if you look at the hourly rate for the studio, you'll probably notice a correlation. You're really paying for the time it takes to make a fully QC'd, verified, and guaranteed master, not the physical media itself. The clock may stop for the hourly EQ rate when the engineer finishes twisting the knobs, but the time it takes to create the master is still figured in.