The problem that I have with your question is that I'm having trouble what you mean when you suggest you base pricing on "number of users/questions". What does that mean? To me, as a webmaster, I start thinking, "Wait - do I have to know how many users are hitting my website or do they mean how many users can manage/edit the questions/answers?" It's a bit confusing.

I'm going to assume that you are thinking along the lines of "How many users the company appoints to edit/manage the entries." Okay - if that's the case then what happens if a big company (like MSFT) decides to try you out on 10 pages that have 1-2 editors? You might get tens of thousands of hits but still get the same revenue that a one-person site with 1k uniques a month gets you. Can you scale that?

I don't know - it's a tough slope but I think you have to put something in there to prevent DOS to other clients because you are fulfilling one big client's requests ten-fold over the others.