http://calculator.s3.amazonaws.com/calc5.html? is a cost calculator.

For only S3:
Storage: 80 GB-months
Data Transfer-in: .5 GB
Data Transfer-out: .5 GB
PUT/LIST Requests: 100 Requests
Other Requests: 100 Requests

comes out to $12.16/month.

It appears S3 is inexpensive. EC2 could get costly.