I sell a 12 talk audio sales training programme - and give away one of the talks as a sample donwload and 2 free talks when I give away sample CDs. This is from the point of view that my prospect has to test the 'tools' in their own experience; that the value of our product is in the way it makes information usable, not in the cleverness of the information.
So for us, one 15 minute free seminar (1/12th of a complete process) is enough for someone to audition on their commute; to pick up a useful insight and to put it to work that same day.
Andddd..... I could be totally wrong since this is the first such product we've produced!
Ref. Snowman's point - it's inexpensive for us because it's digital content.