I was looking for something similar; something that would allow me to use WordPress as a paid-content, subscription-based newsletter platform.
I spent a lot of time researching and evaluating the few systems currently out there but couldn't find anything that was really a good fit for this need. So I developed my own plugin - Suma.
The plugin replaces the content of Posts, Comments and Pages when viewed by a non-subscribed visitor with a notification that a subscription is required. If the visitor wishes to subscribe then the plugin guides them through a typical 4-step registration process:
1. collect username/password details,
2. collect payment information,
3. offer user the chance to confirm the purchase they're about to make, and finally
4. indicate whether the transaction was successful or not.
Once subscribed they can manage the subscription as well as see their transaction history etc. There are also some nice features for the administrator too such as an event log and subscription management forms.
It integrates with PayPal for the payment processing and can accept payment by PayPal account or by credit card.
You can see a demo version of it running on a vanilla WordPress installation at Demo website. You can also download the same demo version from my Online Order System.
Unfortunately I'm not able to provide the production version for free since it took me many months to develop and I'm an independent software developer who needs to eat and pay the bills. However its reasonably priced considering you can make real money using it.