Using options --dry-run and --print-stats would probably give you this information
''Don't really create an archive; just simulate doing so. The list of paths added to an archive (if the -v option is used) and statistics printed (if the --print-stats option is used) will be identical to if tarsnap is run without the --dry-run option.''
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.
Re:Tarsnap price cut
by keeperofdakeys on Wednesday April 02, 2014 @12:39PM
(#7516799)
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.
Been looking at tarsnap for a while but never gotten around to actually try it and my quick naive calculations for backing up my ~ makes it sound too expensive for me even though it's probably the best alternative i've seen so far.