Originally Posted by midnightwriter
No, no, no!!!!!! Unless it is a no profit that you're passionate about, and even then, better just donate your time!!!
OP, I have no experience with non-fiction, but try thinking about it in terms of how much per hour. I'd advertise myself as $15/h, at least, but I would accept a lower rate in the very beginning.
Also, explore profiles of other writers, some of them might list their rates...