At the recent INDA talk I gave, someone in the audience asked about using the paypal integration in DotNetNuke to build subscription based sites. I had to hold my hands up and admit I'd never used it, and said I'd try to put together some notes and post them. However, I see that one of the other coreteamers (Scott McCulloch) has done the legwork already, and published a comprehensive article on this at his site here .
Note: He's recently changed his site to be subscription based (but still has lots of great free content), and judging by his past quality work, I've no doubt that the small subscription fee would be well worth it for anyone interested in DotNetNuke.