Another way to look at it is that if you retrieve data as fast as possible -- say, to S3 or EC2 -- it costs about $1.80/GB, minus the free quota. So for large objects, it's cheaper to store them in Glacier than S3 if you don't expect to need them in a hurry any time in the next 18 months. The free retrieval quota starts making a difference if your typical "ASAP-retrieval" size is less than 0.1% of your total archived data.
Another way to look at it is that if you retrieve data as fast as possible -- say, to S3 or EC2 -- it costs about $1.80/GB, minus the free quota. So for large objects, it's cheaper to store them in Glacier than S3 if you don't expect to need them in a hurry any time in the next 18 months. The free retrieval quota starts making a difference if your typical "ASAP-retrieval" size is less than 0.1% of your total archived data.