There are certain types of wool which are much cheaper, and which will not negatively affect the end product. Sometimes I enjoy working with a little higher grade, such as a Merino wool, because it is softer and a little easier to work with, and it gives your sculptures an even finish and a silky feel. That being said, if you work a low quality wool for long enough, it will look just as finished. Sometimes it is a good idea to use your rougher wool for the base of a sculpture, and the nicer, smoother stuff for the outside and finishing touches.