I've saved rather a lot on the discography discount promos that artists will offer from time to time. When I like someone's output it's an excellent way to expand a collection/taste in one purchase as I can hit play and rather than listen to one or two albums have hours of similar content. Especially for genres I wouldn't normally think of buying it can be an attractive deal (such as recently with some superb electronic ambient/glitch and house from one artist, listened to it for weeks).