Compared to what you pay for the hardware and a support contract and the OS license, it is practically free.
But I don't understand why IBM doesn't just run z/VM on a mainframe and allows hobbyists to run their own VM with an operating system of their choice. Doesn't even have to be free! Five or ten bucks a month for one or two vCPUs and a GiB of RAM would be acceptable to me.
I get that due to the high profit margins[0], mainframes are a real cash cow for IBM, but it's sad they complete ignore the low-end market to preserve those margins.
[0] Last I heard, the mainframe divisions generates 10% of IBM's revenue, but 25% of the profits, so those margins must be juicy.
This mindset is changing in the recent years. There is more information how to hack z/OS than ever, and IBM is trying to fix the weaknesses (for example, discourage custom authorized code, scan existing customer and vendor applications for vulnerabilities, etc.). So they are realizing that obscurity is too little protection for a determined attacker.