I looked into using S3 for my backup software but I ended up going with the vendor that sells the software. It's 1.5 cents more per GB but I don't have to pay for requests or downloads.
S3 pricing breaks down like this
RRS = Reduced Redundancy Storage, it's a lot cheaper but a lot less secure. Good for data that can be replaced if needed, NOT for critical data.
First 1 TB / month $0.125 per GB RRS $0.093 per GB
Next 49 TB / month $0.110 per GB RRS $0.083 per GB
Next 450 TB / month $0.095 per GB RRS $0.073 per GB
Next 500 TB / month $0.090 per GB RRS $0.063 per GB
Next 4000 TB / month $0.080 per GB RRS $0.053 per GB
Over 5000 TB / month $0.055 per GB RRS $0.037 per GB
PUT, COPY, POST, or LIST Requests $0.01 per 1,000 requests
GET and all other Requests † $0.01 per 10,000 requests
All data transfer in $0.000 per GB (Great for backups!)
Data Transfer OUT
First 1 GB / month $0.000 per GB
Up to 10 TB / month $0.120 per GB
Next 40 TB / month $0.090 per GB
Next 100 TB / month $0.070 per GB
Next 350 TB / month $0.050 per GB
Source:
http://aws.amazon.com/s3/#pricing