Tarsnap has always been expensive. More than a decade ago (April 2014, to be precise), @patio11 suggested that tarsnap should increase its pricing. [1] Here’s the HN thread on that post. [2]
All the granular calculations (picodollars) on storage used plus time are fine. But tarsnap was always very expensive for larger amounts of data, especially data that cannot be well deduplicated.