It should be noted that even forums with many active members have struggled to bring in cash through ads.
That's the only issue I see in this.
The main reason for paying people to post (or earn forum cash) is to get fast posts, at least that's how I see it.
However, this business model relies on the admin(s) paying out cash - Which is either out of their pocket, or from their income.
However, even with a large amount of members, the amount of cash you'll be paying out will exceed the amount you will make from advertisements.
So unless you find another way to bring in revenue, you'll be paying members with your own money.
----
To expand upon your idea, I'm curious as to whether it might be worth it to not just have members post on YOUR site, but on other people's sites as well. Let's theoretically say that someone will pay $1.25 for 10 posts.
So if an admin of some other forum wanted posts, they would go to you. They'd pay you $1.25 for each 10 posts, and you'd keep $0.25. The other $1 would go to the member that makes 10 posts on that admin's forum.
This business model still relies on admins paying for posts, but it is more reliable (as far as the amount of time it can be stable for) than the one you propose (UNLESS you find a way to make money, or are okay with givin out your own cash).
For the user - it won't matter (that much) where they are posting. They'll get paid.
For you - You get posts on your forum, and you also get a cut from all transactions.
For other admins - They may have to spend more, but they do not have to deal with finding paid posters. You handle that for them.
Everyone wins.