Responsive image libraries like imgix.js abstract away the complexity of targeting your images at the ever-increasing number of web-enabled devices. However, using solutions like imgix.js can trigger the creation of a lot of new derivatives to satisfy every variety of device.
Fortunately, imgix’s pricing is designed to encourage this behavior, not discourage it, by pricing against origin images and not the derivative images that are created. Using imgix.js is actually more likely to reduce your costs with regard to bandwidth by more efficiently sizing and formatting your images to the specific devices on which they are being viewed.