Assuming you're interested in even compensating yourself for the time you're spending on this, I wouldn't go too low. Maybe base price of $19, and free when you

Assuming you're interested in even compensating yourself for the time you're spending on this, I wouldn't go too low. Maybe base price of $19, and free when you order the physical book? That would still give you some wiggle room to do sales and give discounts to people who ask. Many people will happily pay that much, and if you were to go as low as, say, $9 (closer to 8 after transaction fees), that's really not that much, and you'll need to sell a lot to really make money and be able to keep doing this.