To store two replicas of each piece of data, you must receive the data at one replica, transmit it to the other replica, and receive it at that replica. The data goes in at one server, then back out, and then in at the other server. To store 1 GB of data, you must pay for 3 GB of data transfer. Data transfer is expensive.
Amazon works around this problem by building data centers in clusters, interconnected with low-cost connections. When you upload to S3, your data goes over the Internet only once.