Best way to go about this is sending a mail to certiport. They will assist you further in looking for the testcenter you use and what they will charge you for.
Go to there website www.certiport.com and mail them from there. They will be more than willing to assist you further. As far as I know the exams are charged by exam and not by volume but maybe you can try to negotiate
a deal. I'd say try and see what they offer you.
Maurice
Maurice