docs: update the pricing vision document

api/docs/design-doc-pricing.md

@@ -1,5 +1,31 @@
 # Pricing 
 
+Language Learning App doesn't like that everything has become a subscription, and wants to offer users different ways to pay that suit them.
+
+The exact pricing structure of Language Learning App has yet to be decided, but it costs money to run, and offers value to the user, so it won't be free to use.  The use of generative AI incurs costs as the users use it, whereas other costs are trivial (e.g. creation of flashcards).  Other bits of content (e.g. evergreen topics) could be shared by many users, so access to them could be cheaper.
+
+## Ballpark figures
+
+Users could pay as little as $40/yr and get access to unlimited flashcard creation and testing, and get access to the (e.g. weekly) evergreen content in their language.  This provides *some* way to access content in a language they want to learn.
+
+I would expect a minimum spend per user of $5/mo, and maximum to be $50/mo.
+
+I would expect to see at least a 100% mark-up on the use of AI systems to generate personalised (i.e. user-specific) content, so e.g. if it costs 10p to generate some evergreen content, the cost to the user should be 20p.
+
+For shared content, the cost to each user should be (roughly) the cost that it took to generate, i.e. 10p generation would cost 10p to the user.
+
+## Pay-as-you-go
+
+Each type of action in the system has an associated cost, which will be modelled in credits.  E.g. the generation of a Choose Your Own Adventure could consume 100 credits, the generation of a set of flashcards for a specific word (with spoken text-to-speech) could cost 1 credit.  These numbers are made up.
+
+Users can purchase packs of credits to their accounts, and then as they engage in specific activities, their balance goes down.
+
+## Subscriptions
+
+For users who want the more predictable pricing, there should be an option for subscriptions, with tiers. 
+
+Users who *only* want flashcard content, and text (for example), could be on a cheap plan that allows e.g. 1 choose your own adventure entry every day, and the generation of 150 new flashcards.
+
 ## Ideas
 
 - Dynamic subscription based pricing where the floor is lower if the learner commits to more learning activity.  Price paid at the end of the month decreases for each day where the learner does a certain amount of activity.