Hey,
I can't really advise you on how to structure your forum with regards to how to structure the site. However, concerning paying people NP$ to post, I'd say 1-2NP$ per reply, 3-5NP$ per new thread and $6-10NP$ per registration is pretty reasonable. If you want a better idea, check out the NP$ Incentives forum here at NamePros - quite a few people are paying NP$ to have someone post on their forums, so that should give you a general feel for the price range you'll be looking at.
Also, a quick hint - when you stop paying people to post, most of them will leave. It's something you should be prepared for - if they come to enjoy your forum, so much the better, but chances are, they're in it for the money. However, the trick is to keep paying them for long enough for new non-paid members to think that there's enough activity on the forum to make it worth their time to join and post. It's like opening a new restaurant - the thing to do in your first week is get your family and friends in there and offer them free meals, just to give the impression that there's high demand for your service.
If you build it and populate it, they will come.
Sincerely,
George
