Is it not possible to serve ads to non-members? Let them read-only with ads. Disable downloads. Limit them to the first page of thread only.
Many newb questions are answered quickly within 1 or 2 replies. It is the advanced topics that take more conversation, so to me that is the distinction between paid and free. Free assumes you are a newb and gives you limited access. Paid is if you want to go deeper, have more complex questions, or want to share your own insights.