Sorry, I didn't really define my question well. Basically by users I mean people that may edit the website.
You have really good arguments. I don't really find this being fit for big companies, but I suppose one way of pricing would be by how much traffic your site gets which sort of directly imposes how much money your business is making. In my opinion this is by far the best way to go.
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.