Amazon S3 with reduced redundancy will cost about $45.00 per month plus the cost of the initial data transfer in. EC2 with a reserved Linux micro instance and 500 GB of EBS storage will cost more at about $60.00 plus the cost of the initial data transfer in.

A couple of months agao, I went through the same research as you and it is hard to beat S3 storage costs and ease of use. My requirements are more modest at about 100 GB per month though.

There is a handy Amazon calculator here: http://calculator.s3.amazonaws.com/calc5.html