The answer is you're not really paying for the storage. You're paying for the servicing. The application with its network connectivity and data transfer, plus the overhead costs of billing and support when necessary.
The proposition may look like "$100 for 10 TB", but that's not a ratio that you can halve or whatever. It's $100 for 10 TB and all the network servicing and support apparatus, which doesn't change if the storage amount does.
It's not that Dropbox couldn't offer a tier for less than 100/year, it's that they don't want to. We around HN know about reducing headaches by avoiding pathologically cheap customers. Dropbox makes more profit overall by getting some of the would-be cheapos to round up to 100/year and letting the rest walk.
