I'm taking a B&W class and my instructor said that it would be a good thing to invest in filters because the ones at school may not be the best. The way they maintain their sets is that, if one filter is lost from set A, they will try to get one from another incomplete set. Besides that, they're old and they've been used to death. Anyway, I wanted to know which kinds are the best and how much should I expect to pay. I hope they're not too expensive; I would like to get a set. Thanks.