Missed the dry-run option but it still seems to require a key which requires you to pay. Talked with some on irc and it seems it's not possible at the moment. Someone mentioned gzip providing a good enough estimate on the compression without taking the de-duplication into account.
Gzip would provide a fine-enough approximation, since both that and tarsnap use zlib. Tarsnap also has deduplication, so equal (or almost-equal) files will take up much less space.
