Since some months, Amazon's S3 has a so called Reduced Redundancy Storage (RRS) option. It has “only” a durability of 99.99% (instead of 99.999999999% in regular mode), but it's 33% cheaper. If your data doesn't need almost 100% guaranteed durability, because you have stored them anywhere else or can recreate them easily, that's a really money-saving option.
It's also a viable option, if you use AWS mainly as CDN. As we used s3fs for 72h.ch and we wanted to use RRS there to save some bucks, I patched s3fs and added support for RRS.
To tell Amazon that you want to store your object in RRS mode, you just have to send an additional header, therefore the patch was just a few lines. You can get it on my s3fs-fork at github.
To use it, just add “-o use_rrs=1” to your mount command and from now on all new or changed objects will be stored as RRS option.