The guys know better than me but with all Amazon's price reductions, 100Gb/year comes to $44.40. There are transfer costs on top of that however there are no longer transfer fees IN to S3 and I would guess that the OUT bandwidth is primarily generated by a small number of files on your machine and probably stays pretty linear past a certain point.

I think those prices were the cheapest viable at the start but it feels like there are more competitive ones now (and that's assuming they're not building their own cheaper datacentres).