Field paywall (https://www.drupal.org/project/field_paywall) is a Drupal module I produced for News Corp Australia that allows Drupal editors to assign paywalls to entities choosing which fields to hide and what kind of message to display. Using the core Drupal Field API framework, Field Paywall can be retrofitted on to existing nodes without having to make major changes to system configuration as all configuration is done through field instance settings and is Features exportable.
The following video is a walkthrough on how to get started with Field Paywall, enable it on an existing content type and theme the paywall message.