The plan would also provide you with any upgrades released during that time, all for one price. I'd say all for one low price, but we don't know how low it will be since Microsoft isn't saying. My bet is they've yet to decide subscription pricing. It's no wonder that Microsoft is taking its time to make a considered decision. Monthly credit card billing isn't available.
Subscriptions ought to be fairly inexpensive.
The total number of subscribers should be much larger than the number of legal copies sold today. The estimate I usually hear is that there's at least one pirated copy for every three software programs in use. But for Office, with its enormous market share, I'd bet that number is conservative. Still, if the number of legal users increases by a third or more, Microsoft could reduce the price by that much and still break even. Or maybe not.