Susan the subject matter is not as common as say portraits and gets you a higher price, maybe the buyer recognized the photographer, the case is another thing that can bring money. Since you've said plastic the case would be what people call union cases and they were made from Shellac saw dust and other stuff. Those early thermoplastic cases are often worth more than their content. Good literature would be: Union Cases: A Collector's Guide to the Art of America's First Plastics by Krainik and Walvoord