Until they switch backends, or negotiate with Amazon. While google has the buying power and made investment in running cheap, reliable datacenters, DropBox has so far made its investment in good software and user acquisition.
Dropbox has a number of ways out of this, and they've got a healthy user base and plenty of access to VC dollars to fix this.
The S3 price is for unique storage. A lot of content is going to be duplicated across Dropbox users so they won't need to pay S3 for every byte, just the unique ones.
1 TB on Amazon S3 = $125/mo (down to $55/mo for 5,000 TB)
Being based on S3, Dropbox is going to have a tough time competing on price.