(caught 'assuming', trying to pull pants up quickly ;-)
No, you're right - my basic arithmetic comes out with the same number, now that I've checked it. Good catch.
I assumed (thwack) that, since 'tarsnap' is only taking a small premium over Amazon S3 charges for real use, it would always be a better price. Wrong (thwack), if you do have between 30 and 50 GB.
Possibly Dropbox can beat the price because the buyers of the 50GB plan use less than half, in average. (Leaving aside considerations of Amazon vs. dedicated infrastructure costs.)
