I'd expect one either pays for the space used at whatever the going rate was when the year elapsed, or whatever rate was locked in when one entered this agreement, or one chooses to lose data exceeding the amount of space one is willing to pay for. This seems quite straightforward to me and fair.
But what is neither straightforward nor fair is what one always has to look out for -- impossible "unlimited" storage promises because they're always a lie. There isn't unlimited storage available. So it's always a matter of nailing down precisely what limits will be set up to restrict your use. And, as a side note, be prepared to defend your use against anyone who wants to claim you're "abusing" the space merely by making the space hold "too much" or transferring "too much" data when you're buying into an "unlimited" storage space.