I occasionally read the techexams.net forums for tech certification deals and this is one of the discount voucher sites that is commonly cited there. It is probably not in Microsoft's best interests to be transparent in pricing because of all of its business accounts and third party voucher systems allow it to be opaque.
Note: There also seems to be a system for student and teacher pricing on these vouchers. More research here might result in an inexpensive voucher.
Note: There also seems to be a system for student and teacher pricing on these vouchers. More research here might result in an inexpensive voucher.