tarsnap --dry-run --no-default-config --print-stats --humanize-numbers -c /MY/DATADIR will tell you the compressed size of your deduplicated data, which gives

tarsnap --dry-run --no-default-config --print-stats --humanize-numbers -c /MY/DATADIR will tell you the compressed size of your deduplicated data, which gives you the upload cost and first-month cost. 4GB of files usually works out to 3GB of dedup/compressed archive data for most people, less for people with many similar files.