How about a model that credits members when they contribute and that contribution is acknowledged by other members. For example (nb: you might need to analyze and tweak the numbers):

1) Each time a member gets awarded 1 star (a help star) by the community they get CAD1 added to their credit balance.

2) Each time a member gets awarded a solution star by the community they get CAD5 added to their credit balance.

After 1 year member must subscribe to continue to have access to the site. But his subscription is normal retail price less credit balance earned. If a member has contributed a lot his renewal fee could be zero of even a credit balance which can be carried over to the next year.

New members always get the first year free in order to give them time to build a credit balance (or not depending on their type of activity).

All existing members can start their year at a designated date for all existing members.

Those that mainly ask questions obviously don't earn credits but are getting advice for free.
Those that mainly answer questions get free membership via the credits systems.
Those that are a combination of questions and answers will get some credits.
Credits are awarded by the community.

Obviously some members who answer many questions will never pay but they are worth having in a community because they are so smart. The leaches, like me, must pay :)

In this way you are effectively getting members who give their knowledge a reward for doing so and it is being paid for by those that need the advice.

Like I said at top, you will probably have to analyze the numbers and tweak them.