Don't use separate portals.

Instead create a user role specific to each course, then create the different courses as different site sections/tabs. Then apply appropriate view permissions for your roles to those sections. Most payment processing modules allow you to automatically add the user to a role upon payment. Even the built-in DNN subscriptions work like that. You can also add people manually to roles - use that to handle the people who have paid in advance.

Rob

You can use permissions to decide which tabs are viewable.  You could "theoretically" add the ability to view tabs based on which course has been completed, bassicaly different levels of permissions based on what has been completed on the site, it should be possible but I am not sure if there is already a module ou there that does this.  Having a site of 15 courses that are 30 pages for each course presents a bit of a challenge but it is a normal challenge for a lot of pages.  An example solution would be that each course is in a different major catagory and so you are using sub catagories for the actual course list, you could use child links etc.  Keeping all the information on one portal seems to make the most sense with what you want but if you want to go with more than one portal you can because there are modules that allow you to share user names and passwords across portals, just look up user registration modules on snowcovered.