Look at the typical chain of product movement in offline retail and online retail.

In offline retail, there are more handshakes than online, and cost increase with every incremental handshake. Typically, final price at which a consumer purchases a product is 15-25% more than the selling price by a manufacturer/marketer. In online retail, since there are less handshakes, the benefit is passed to the consumer, thereby acquiring a customer and making him/her happy.

Sometimes there can be predatory pricing due to competition/customer acquisition, but all in all it's good for the consumers.

Disclaimer: The retail chains shown above are typical model and may vary with different manufacturer/marketer. The number above is just for explaining purpose.