Keep money per answer out of this. It's amazing what happens when you keep a site like SO working on social norms and avoiding market norms.
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.
Some experts are not only in for the money - they enjoy helping the novice, the less experienced. They're not only in for answering only the most interesting "gems" - a true expert can give a much better answer to a newbie question than an intermediate developer.
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.