The only cost now is hosting the website at S3 and those costs are substantially lower than competitors. NearlyFreeSpeech[2] is the closest web host to S3 that I can think of and their prices for storage are $10/GB ($0.01 per megabyte month) whilst S3 starts at $0.140 per GB. If your site gets hit incredibly heavily Amazon S3 will also handle the load transparently. If you find that your site is becoming popular in Europe or Asia than it's also supremely easy to push your site from S3 to CloudFront.
I wonder how far you can take this though? How complex a site can you set up using only static hosting and external dynamic services like Disqus, MailChimp and Docs?
[1] https://github.com/mojombo/jekyll/wiki [2] https://www.nearlyfreespeech.net/services/hosting
[EDIT] As TeHCrAzY said I ovestated the cost of bandwidth transfer but whilst bandwidth falls rapidly it still falls slower than S3 (at ~10TB of transfer it approaches S3's starting price). More importantly I realised I undestated the cost of storage - it's $10/GB and not $1/GB making it closer to 100 times more expensive than S3.