The WWC is free for volunteers. The requirements for obtaining one is not the same in every state in relation to religious volunteers. The WTBTS in Victoria only started complying to some degree after the efforts of a hardworking guy called Steven Unthank. His efforts appear to have had the direct effect of the Kingdom Halls changing their signs to remove the Ministry School wording.
Fortunately it also resulted in the organization advising Elders to apply for a WWC card, at least in Victoria. As to how the Branch Office monitors this, or anyone else for that matter, is unknown.