They certainly can be. Because of the desire to be "screen accurate" and to act the part - and not just dress up for a night - it can be an expensive hobby. However, it doesn't have to be. While some costumes can cost you over $1,000, there are plenty of good ones that will run you more in the $200-$400 range. If you don't care as much about the types of fabrics, you can possibly go cheaper as well. No matter which way you go, don't forget the extras such as shoes, props, wigs, or masks, that don't always come with the initial outfit.