I'm in the process of trying to get that information for you. I do know that the pronounciation of Costner and Cosner I've only heard one way but the pronouciation of Kastner, it ometimes is prounounced a couple of different ways. I think tht is why the name is spelled so many dfferent ways here. (Casner, Cassner, Kassner etc.)

Do you know the movie star Kevin Costner. If you've heard his name pronouced that is the most common way for Costner.