It's cheaper than their nearest competitor in the space: http://www.box.net/pricing/
So on the one hand, you have DropBox with unlimited storage and a limited, upgradeable number of users. On the other hand, you have Google, with limited, upgradable storage, and unlimited users.
If only Google's storage was a bit more usable. (Also, I have a hunch an AWS-based solution may be cheaper depending on some other variables.)
Like I said, the problem is not so much the package and price as it's the fact that it's the only package they're offering right now. I see very few use cases in practice for unlimited storage and limited users.
EDIT: s/unlimited storage/200 GB per user/ where applicable