Pay a real money fee to retrieve the stored things? I feel like the storage requirements have been exaggerated a bit. Let’s take a random msgpack file from the boundless files, compiledblocks.msgpack. It’s 4 megabytes in size, and it contains over a million rows of data. I doubt the game has that many unique items/blocks the server would have to store in the worst case scenario.
Even if 100 000 players let their stuff expire and they all had a million unique item types in their builds, the total storage required to store that is 400 gigabytes. And that’s probably an order of magnitude bigger than what it would really be.
Having the customer pay a restoration fee sounds a bit excessive 
But that option seems to be less popular anyway so probably not something that will be relevant.
The size isn’t trivial for sure, but it’s a small thing in datacenter standards.