It would seem to me that naively, S3 charges $5 per million POST requests, so it's 10x worse than SQS's $0.40 per million.
Any middle tiering of the data before it reaches the consumer, is still "the queue". You don't need to know the internals of SQS, anymore than a consumer need know the black box elements of how you collate the messages within your ad-hoc queue.