At TeachMac we generally price based on the number of lessons. 10 lessons or fewer are generally free, 11-20 $5, 21-30 $10 and 30+ are $15. Some factors to consider is how long is each individual lesson, how detailed is the text, is it a consumer app (lower price) is it a professional app and you're helping someone make money (higher price). Ultimately, your own judgment of the quality of your product is your best guide. Later we'll add a rating system and that will help guide everyone.