Audio-Technica uses high quality materials that are chosen to ensure comfort during use as well as provide the audible performance expected from our headphones; this is by creating a reliable seal to prevent sound leakage and allow as much of the intended sound to reach the ear canal as possible, and the material allows us to ensure the drivers are a specific distance from the ear canal which affects the tonal response.
Although there are 3rd party ear pads available from other vendors at a much lower cost, the materials will be very different and may not enable the product to perform at its best.  For this reason, we wouldn’t recommend using these alternative pads, to ensure the headphones are able to perform within factory specifications; however, we understand that the price of our ear pads can seem high in comparison and so the final decision is yours to make.