[Amazon S3 charges] $0.15 per gigabyte of storage used per month, plus $0.20 per gigabyte of data transferred. That's a microscopic price, even compared to the charges at hot new online storage providers like Box.net ($1 per GB per month) and XDrive ($2 per GB per month) -- not to mention older providers like Novastor ($12.95 per GB per month) and Quicken Online Backup ($24.95 per month for up to 10 GB).
Wade points-out that if you do a lot of transfers that the transfer charges might add-up and make the Amazon option less of a guaranteed cheaper option. He also notes that other big players, Google and Microsoft, are likely to weigh-in with low-cost online storage offerings of their own before much longer. Yet Amazon has lead the way on this front.
As Dion Hinchcliffe pointed out it won't be long before you'll be able to dynamically choose which storage service provider to use.