Kosher chicken is usually more expensive. But there’s a reason. Health is a major factor in a chicken being kosher. It cannot have any health issues that would render it to a non-kosher status of a "treifa". This includes certain sicknesses and lesions or holes in vital organs. The most common problem is that of lesions in the lungs that need to be checked by a qualified rabbi to make sure it is not a "triefa" and forbidden to eat.
Also a kosher chicken needs to be ritually slaughtered accorded to Jewish law. This is called "shechita".
The meat also needs to be washed and salted in accordance with Jewish law in order to remove virtually all of the blood.
And all of the above needs rabbinical supervision by people who are specially trained and are diligent in making sure that all is done to ensure that the final product is “kosher”.