Books currently sell more than courses, but in our opinion, courses should be priced higher than books. The ideal target for courses are people who are using a company credit card.
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So, for an arbitrary Leanpub course, the price should be about 2x - 5x the book price, or between $49 and $249 based on the subject matter and the depth and amount of material covered. For example, our bestselling course (https://leanpub.com/c/stratospheric) has a minimum price of $249.
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So, for a typical course based on a book which was priced between $20 and $40, I'd recommend starting by having a price of say $149 or $129, and giving away discount coupons in your promotions to make it say $79 or $49. Then you can test both the high price and the discount coupon price at the same time, and iterate from there...