You could factor that into the price, but then you're potentially making the price point even more unattractive to users than it already is, and users that are responsible with their budgets would be subsidizing those that aren't. Not a very workable solution.

I'd say a good solution is giving customers the option to stop accruing more storage capacity, and to have a max deadline accounted for in their budget to store data (basically each customer decides whether or not to pay for a grace period).