Zipbooks - has time and project based invoicing, reminders, payment integration and simple to use

its more about what they can make from the contents of the ebook. If it's knowledge- and resource-packed, that would take a lot of time to find or discover otherwise, then you can def upcharge accordingly and it would be a fair price.

key aspects being - info not (easily) available elsewhere, pitched and targeted to readers who are actively seeking the solutions you talk about (they're in the sales window), and reviews/testimonials of others talking about how this helped them.

we did a real estate industry focused one which had 1700+ purchases at $80 each, and another one for the securities investment folks who paid $700 for that.

so "fair price" depends on the info, industry, buyer.