Greg,
Maybe a glitch in CentOS or YUM made the bill higher than it needed to be.
I recently moved two systems from CentOS 3.1 to 3.3. On both machines, I started with yum.conf files pointed at my own mirror. On both machines, all I did was "yum update", but my yum.conf was renamed, and I got new yum.conf files pointed at mirror.centos.org.
I was watching this carefully only because I migrating these two systems in part to test my own mirrors. If not for that, I might not have noticed the switch. (And because I am testing my own mirrors, I got rid of the new yum.conf files and put my own back immediately.)
Maybe lots of other sites got switched to mirror.centos.org and they have not even noticed. Or they assumed the upgrade switched them because cAos wants them to update off mirror.centos.org, so they left the new yum.conf as is.
This could be the main cause of the big bill.
Rick