$5-$15 per answer would be reasonable given the limited time involvement.
It's doubtful to me that you could appeal to expert users to exchange time for money when it is better leveraged by consulting and entrepreneurship.
I seem to have enough free time to blow a fair portion of it commenting on Hacker News.
To understand how powerful this difference can be, this book is an eye-opener (outlined here):
Other than money, how do you prevent an expert-exodus as expert users are deluged with simpleton questions? Amidst a deluge of bad questions, there's little value to remain active, as your own questions can rarely be answered, and users aren't providing interesting questions.
Something else is needed to provide a substitute for that value.
Alternatively, you must prevent the exodus by filtering the kinds of interactions/questions that occur. Mailing lists, for instance, have a barrier to entry to serve as a first-pass filter (the subscription), and then a community to enforce community norms.
Similarly, (most) university professors don't teach for getting rich or just fort the research - they enjoy overseeing and helping the youngest generations of their particular field.