With an active subscription, you'll pay 14.99 USD/EUR (for Flow® Mastering Suite or Flow® Mixing Suite) per month and earn 30 credits, or 24.99 USD/EUR per month (for Flow® Complete Suite) and earn 50 credits. That means for every USD/EUR you spend on your Flow® subscription, you earn 2 credits. Therefore, you are effectively receiving a 50% discount on the list price of the plug-ins found in the Flow® Credit Store.
If you have purchased all the plug-ins that make up a Flow, you essentially own that Flow. Even if you cancel your Flow® subscription, you can still use that Flow within the Flow plug-in.