Well, regarding created two unique discussion pages entirely, the short answer is you can't. But there are some things you could likely do to get the same effect.
Using the same discussion page as you already have, you can actually set up each discussion category you create to have a viewing and posting access level. This means, if people don't have the right access level, they can't post or view in that category. You paying people would need to have an access level higher than 1, and then whatever access level you choose for the pay discussion categories will need set to that level too.
In order to change the access levels for each category, you can do so a number of ways. You can do so during the time of creating the category in the options provided, or you can also do so if the categories already exist by going to your discussions and looking for the little edit link located on each category.
