My module doesn't have that feature. Sorry. I guess you'll need to save the product price info with the subscription info so you can keep a record of the specific customer's price. You could probably add a "renewal_cost" column to your "digital_subscription_customers" table to provide that info. If you need any advice as you work on your module, feel free to ask more questions under this topic. If you start another topic and you want my input, just send me a personal message with the URL for the topic.
Getting back into x-cart dev after a long hiatus. Modded lots of x-carts from version 3.1.x to 4.1.x. Developer of ImageScaler mod, Pre-login per user coupon mod, Wordpress feed mod, DigitalSubscriptions mod, Phonetic bulk download keys addon for DownloadExpander mod, Serial Number Generator for ESD products, Custom CMS/LMS integrations, external products mod, and more.