Compared to the cost of building and maintaining your own system I think it is cheaper. Add the cost of servers, colocation, bandwidth, people.

Also, on S3 you pay exactly for what you use, if you use your own server you are paying for them up front. Another bonus is the using their CDN if you have frequently downloaded files.

Check this comment by Don MacAskill, CEO of SmugMug which stores a huge amount of data on S3: http://news.ycombinator.com/item?id=422574. Check his blog too: http://blogs.smugmug.com/don/tag/s3.