Per gigbyte of what? Not certain what you are talking about; in normal Slashdot style I will answer your question anyway.
It is standard practice in many large corporations for departments to pay ‘charges’ for the infrastructure and supplies they are using. Not real money, just numbers so that the bean counters can figure out what stuff costs to get things done, and to juggle the numbers to make things look better or worse (as directed by their managers).
At one place I worked they charged for server disk space. The theory being that it cost money not only to buy the disk or make it redundant, but also to back it up incrementally, offsite forever, transport it for me through the network, process the data on the servers, pay the IT staff to support me, to be trained and to go on vacations. The number usually was much higher than you’d expect, and actually included *all* IT expenses. They used disk space as a *fair* measure of my groups IT needs.
Remember too: some places will backup your PC via the network, so just because the disk is cheap on the PC doesn’t mean that is the whole expense per GB.
I don’t know what it costs these days at a big company. $30/month sounds like an old number for old infrastructure, not your desktop PC. Thank God I work for a small place now and the accounting is almost sane.